Two instances immediately come to mind:
A German pilot escorted a crippled American bomber crew to safety. (http://en.wikipedia.org/wiki/Charlie_Brown_and_Franz_Stigler_incident)
At the very end of the war, after Hitler killed himself, a German unit and an American unit fought together in Austria to defend a castle containing a group of high profile French prisoners from a last ditch attempt by an SS unit at revenge. There's a book about this called "The Last Battle." (http://www.dailymail.co.uk/news/article-2323949/The-World-War-II-battle-Americans-Germans-fought-SAME-Incredible-true-story-fight-save-Austrian-castle-weeks-Hitler-committed-suicide.html)
Memoirs aren't usually the most trustworthy of historical sources but I nevertheless wanted to share a (rather long) quote from German Col. Hans von Luck's memoir Panzer Commander. It is from his time in the desert of North Africa while he was serving in Rommel's Afrika Korps, at the end of 1942. It doesn't exactly amount to a Christmas Truce, but it comes quite close.
We quickly developed a a certain routine. Toward five o'clock in the afternoon the reconnaissance patrols broke off their operations in order to reach base in good time; in the treeless desert with no landmarks it was impossible to find one's way back to base in the dark. To avoid betraying our position, light signals were only used in an emergency. The two British battalions carried on in the same way, so that from 1700 hours, all reconnaissance and combat activity was suspended, to be resumed again in the following morning as soon as it was light.
"We could really agree to a cease-fire with the British from 1700 hours until morning", I said, more as a joke, to those around me.
"Why not?" I was supported by Lieutenant Wenzel Luedecke, the reserve officer who had worked at the UFA film studios as an assistant director. "After all", he went on, "the British have a sense of humour. We ought to suggest it to them."
Chance came to our aid. One evening, when all our patrols were back, I received a visit from my intelligence officer.
"The Royal Dragoons are on the radio", he said, "and they would like to speak to you."
"Hallo, Royal Dragoons here. I know it is unusual to make radio contact with you, but Lieutenant Smith and his scouting party have been missing since this evening. Is he with you, and if so how are things with him and his men?"
One of our patrol had indeed managed to take some prisoners. It turned out that they were Lieutenant Smith and his party.
"Yes he is with us. All of them are unhurt and send greetings to their family and friends." Then came the brainstorm. "Can we call you too, or the 11th Hussars if we have anyone missing?"
"Sure, your calls are always welcome."
It was only a matter of days before we had arrived with a 'gentleman's agreement':
At 1700 hours, precisely, all hostilities would be suspended. We called it 'tea time'.
At 1705 hours, we would make open contact with the British to exchange 'news' about the prisoners.
In fact, from a distance of 15 kilometers, we could often see the British get out their Primus stoves and make their tea. The agreement was kept by both sides, until we were forced by events to give up the connection in Tunisia. The prisoners we took often had to stay with us for several days, until the next supply convoy arrived and was able to take them away with it. We gave them whatever we could share from our rations.
One evening, when our radio stations had tuned in once again to the Belgrade transmitter and we heard the song 'Lilli Marlene', some of the prisoners joined in.
"Over there we listen to 'Lilli Marlene' every evening", they said. "There's already an English version. Monty has strictly forbidden it, but we like the song and its sentimental words." The French and the Americans also listened to it, as we found out later. Somehow, it made things easier.
Our 'five o'clock tea agreement' had some remarkable consequences. One evening, a patrol car came back with two men and a jeep captured in the desert. A tall, fair-haired, young lieutenant and his driver were brought before me. The lieutenant was the snobbish, arrogant type of Englishman. Very correctly, he gave me his service number only, no other details.
I tried to get into conversation with him and told him of my visits to London, of my friends, including a captain in the grenadier guards. He gradually thawed, and turned out to be the nephew of one of the owners of Player's cigarettes. My officers made a whispered suggestion and I had to laugh.
"Lieutenant, what would you say to our swapping you and your driver for cigarettes? We're a bit short at the moment."
"Good idea", he said.
"How many cigarettes do you think you are worth, what should I suggest to your commander?"
His answer came without hesitation: "A million cigarettes. That's 100.000 packets."
My radio officer made contact with the Royal Dragoons, and I passed on our offer.
"Please wait, we'll come back to you at once", was the reply. Then, after a few minutes: "Sorry, we're a bit short ourselves, but we could offer 600.000 cigarettes. Come in, please."
To my great astonishment, I received a flat refusal from the young lieutenant.
"Not one cigarette less than a million, that's final!" was his answer. So the young man had to pay for the high value he set on himself with captivity.
A week later, shortly before dark, our doctor disappeared behind a rise for the indispensable 'spade trip'.
"Doctor", I called out to him, "don't go too far, it'll soon be dark." He didn't seem to hear me and went on.
When he hadn't come back after half an hour, we began to be worried. The doctor was not only very popular; with his tropical experience, he was vital to us. We sent out some men and fired the prearranged light signals. The doctor remained missing. Had he lost his way, or had he been caught by the British?
"Yes, we've got your doctor. He ran straight into our patrol on its way back. This time, we have a suggestion. The Japanese have cut our communications with the Far East. We can't get quinine anymore and are suffering badly from malaria. Can we exchange your doctor for some of your synthetic Atebrin? Come in, please."
"Please wait", I replied.
A moral issue now presented itself: Which was more important, to weaken the fighting strength of the British through malaria, or to get our doctor back? I quickly made up my mind.
"Okay, we'll do business. How many packets do you want for the doctor?"
We agreed at once on a quantity that we could spare and arranged the exchange for the following morning. From either side, a jeep with a white flag drove between the lines for the ceremonial trade.
"An expensive spade trip, Doctor. Good to have you back."
Rommel, to whom I related this on one of his visits, was understanding. "That's what I thought about the British. I'm glad you can practice this fair play here in the desert; on the coast, it's just a matter of survival."
Only once was our 'agreement' unintentionally broken. One evening, a patrol returned to base from its operation with a British supply truck. The leader of the patrol was a young lieutenant who had joined us from Germany only a short time before.
"Major", he reported proudly, "the truck is full of corned beef and other tins, beer and cigarettes."
"When and where did you capture it?" was my first question. It turned out he had captured the truck after 1730 hours, hence after the agreed time.
"Are you mad, you know the arrangement? This will not be the end of the matter."
The lieutenant was astonished. "But these are things that are really useful to us, and which will be denied to the British. War is war."
I had an idea of what would happen and at once sent off a radio message to Rommel. "Have impression that British patrols intend to outflank us in the south. Suggest moving south."
Rommel agreed and sent word that another small unit would take over my position the following day. I briefed the leader of the unit on the situation in my area and warned him expressly against British patrols, which would appear suddenly and try to take prisoners among us. In the afternoon, I moved south.
What I had suspected promptly occurred. In the evening, toward 1730 hours, a British detachment raided the unit, captured two trucks, and disappeared into the darkness. A gentleman's agreement was, after all, a gentleman's agreement.
The end of our agreement came later, somewhere in the depths on the Tunisian desert. For some days, we had lost contact with the two British battalions. Then, an orderly came to my command car one evening.
"There's a Bedouin here who wants to talk to you, Major."
With a deep bow, the Bedouin came in. "Salaam, I have a letter for you. I will wait for an answer."
A Bedouin with a letter, here, depp in the desert, where by rights no one could find us? The Bedouins always seemed to know where we were. I opened the letter.
"From the CO., Royal Dragoons
Dear Major Von Luck,
We have had other tasks and so were unable to keep in touch with you. The war in Africa has been decided. I'm glad to say, not in your favor.
I should like, therefore, to thank you and all your people, in the name of my officers and men, for the fair play with which we have fought against each other on both sides.
I and my battalion hope that all of you will come out of the war safe and sound, and that we may find the opportunity to meet again some time, in more favorable circumstances.
With the greatest respect."
I sat down at the table and wrote a similar note to the Royal Dragoons.
In the book Band of Brothers (and perhaps Citizen Soldiers) by Stephen Ambrose, some of the soldiers told a story where around midnight on New Year's Eve in the Ardennes Forest (Battle of the Bulge), some of the Germans began to sing Silent Night. The Americans soon joined in with them, with some of the soldiers describing it as one of the most peaceful moments in the entire war.
This response was not universal on New Year's Eve: along the same line, one of the American commanders had every single piece of artillery fire on the German lines at the stroke of midnight to "ring in the new year".
There are a few examples of musical interactions during the war. The one that springs to mind was at Bastogne, where soldiers on both sides sang Christmas carols within earshot of enemy lines. There were a couple examples of trumpeters playing music within earshot of enemy positions as well.
I can't recall any open fraternizing between opposing forces like the Christmas truce though (or during the American Civil War), where soldiers would have met face to face.
http://ba-ez.org/educatn/LC/OralHist/vincken.htm -- this is an interview of a man who, with his mother, invited lost American soldiers in for a Christmas meal during the battle of the bulge. When some German soldiers arrived they invited them in as well and during the battle of the bulge, American and German troops shared a Christmas dinner