I have been having a bit of fun with this mod recently, its for wolfenstein ET - so you dont have to pay anything ;D
I recommend you all try it out- if you like fun things ;>
http://www.truecombat.com/intro.php
A summary of the game is that it is basically a Counter-strike game, only with classes and no crosshair. And instead aiming is done through bringing the weapon to your face and using the iron-sights. ;o
Also, you can sprint and prone- and the movement speed is a little more realistic. ;>