Soldat is a 2d game where you aim with the mouse and kill your opponents with one or a few well placed shots. I played an absolute fuckton of Soldat growing up, and now the creator is working on a sequel, and it's a blast.
There's not a whole lot of people playing, and most of them are scary good, but this is your chance to get in on the ground floor! I highly recommend it. The sequel has better graphics and less Polish swear words, but I expect the second part to improve once more people discover it.