Master and Commander does a FANTASTIC job of illustrating living conditions and day-to-day life aboard a sloop on long service in the Royal Navy, and the combat is really realistic on -most- points. Very few gaffes, and most are pretty minor.

Not exactly a movie but the show Rome tended to show fairly accurate Roman warfare when it was shown (I believe the first episode has a great example if your curious). The discipline in the Roman troops was shown as stepping out of line was highly looked down upon. It also highlights the rotations that Roman soldiers used to keep their front line infantry relatively well rested. As well it shows the classic technique that Romans used which was blocking the enemies attack with their shield and doing a uppercut with their sword into the exposed enemies stomach.

In true Kubrick fashion, Barry Lyndon's battle scenes are reasonably accurate. You seldom see a 'full' battle, though. They are a bit more realistic than the 'clean' battles you tend to see in other movies set in that era.

One battle from Barry Lyndon: https://www.youtube.com/watch?v=1cIjTodmfk0

The Soviet production of War and Peace is another good one, although you have to find them amidst all of the plot and 20 minute long single-cut scenes of people talking about marriage: https://www.youtube.com/watch?v=RLovWXcjiZI

There was one particular scene from a Spanish movie called Alatriste that I recall. It wasn't an open field battle but rather a mining operation under a fortress wall. It was memorable to me because they accurately represented the kind of conditions and fighting you would find in a siege. And watching two guys knife fight in a two foot tall tunnel is pretty amusing.

The 'grand finale' battle of Alatriste is amazing. I don't think I've ever seen another movie do a real pike-and-shot battle: https://www.youtube.com/watch?v=tgQDZKP-pmE

Sorry to divert away from op's question, but how accurate was "a kingdom of heaven"? In terms of day to day life in the Levant during the crusades?

As far as a movies, I always thought this scene from *The New World had a unique portrayal of combat that showed a sort of natural ebb and flow of contact, combined with a fair dose of 'what the heck is going on'. Perhaps someone could comment on its accuracy?