Submachine guns were popular war trophies on the Eastern Front. The Germans used captured PPSh-41 and PPS-43 submachine guns in large numbers. A few were even converted to 9x19mm, adapted to take MP-40 magazines, and designated the MP41(r).
The Soviets in turn used German MP-40s in substantial numbers. Partisans obviously used captured MP-40s, and they were also well-liked by reconnaissance troops in particular.
https://imgur.com/a/jTCoNNE - an MP-40 in Soviet use
https://imgur.com/a/KVsU9Oa - a PPS-43 (left) and a PPSh-41 (right) in German service
https://imgur.com/a/l08InYt - a PPSh-41 in German hands
https://imgur.com/a/z0Cv924 - an MP-41(r) conversion and an MP-40 (right). Note the identical magazines.
Machine guns were also commonly-used war trophies. The Germans frequently used DP-28 machine guns and the Soviets used MG-34s.
https://imgur.com/a/DL3dI7v - a DP-28 and Moisin-Nagant rifle in German hands.
https://imgur.com/a/k0hZfVB - a German infantry section with an MG-34 and a DP-28.
https://imgur.com/a/Qsyunbr - two Soviets with an MG-34.
And this isn't even touching on the wide-scale use of captured tanks, artillery, and anti-tank guns by both sides!
When it came to personal items, medals, insignia, alcohol, cigarettes, food, boots, and winter clothing were commonly taken. Soviet soldiers especially liked taking German watches - infamously, the soldier in the famous Reichstag flag photo was wearing two looted watches, so one was editing out by Soviet censors.
