Added 11 minutes later
by the way, my brother downloaded some 2D game called "Soldat"...
there are several game modes: everyone for themselves (the most normal), capture the flag (don't know what it is, played it once) and maybe something else
in general, you customize your soldier and so on, then choose a server (you can create your own) and go to war! as soon as you appear, you take one of the offered weapons (and additional weapons), and then you start shooting in all directions...
it's a pretty cool game, and it doesn't require a super-powerful computer or anything else, and it's also easy to use)