Counter-Strike was super fun during the time it took form (the beta versions from 0.5 to 5.2 ish). They tried a lot or different things, like dual wielding machine guns. The community was awesome also.

Beta 0.1 will always have a special place in my heart playing it for hours at the LAN shop down the street from me. Bonus was being able to troll people by flying my dead-player spectator through doors and opening them. People would get really mad - it was awesome.