Alien Arena is the greatest Freeware of all time.
User Rating: 8.3 | Alien Arena 2006 PC
Alien Arena may not be the best shooter ever, but it is good fun, and completely free. The main game is a simple deathmatch, which can be very enjoyable, with a good amount of weapons and powerups to add to the mix. Some of the weapons, like the alien smartgun, can be cheap, along with the quad damage powerup, but if you get fragged by them, you can get back up and get your revenge quickly. There are a huge amount of maps to play with, and a single player with suprisingly good Bots. AA is a great Freeware game, but nowhere near as good as the standard FPS game. My score is keeping that in mind. 
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