WHICH IS THE OLDEST GAME YOU'RE STILL PLAYING TODAY :)

Killing Floor. Not second, the first. Though it's dying, you still can have alot of fun if you play with your friend or on a modded server, there's a 75% discount in steam, it's worth buying, try it out yourself!

The game itself is like left 4 dead, but instead of multiple map walkthrough a single map with multiple waves. You buy weapons inbetween them and kill zombie-clone thingies. At the end of a final wave comes a bonus wave (8/7 for example), where comes in the big boi - Patriarch. If you kill him - mission complete, but it's not very simple. Also there is objective gamemode where you need to do something instead of just surviving. There's alot more to this game, but im too lazy to explain, look it up yourself :b