Alien Swarm's all right - it's not best game ever, but it gets the job done, and the price is right.
But yeah, I love me some freeware games. Check out
Hydorah (great retro-styled horizontal shmup - think Gradius or R-Type),
Iji (A bit like Flashback/Blackthorne/Out Of This World, with a brilliant story and some great touches to it),
Genetos (vertical shmup with great music and an awesome upgrade system) and
Battle for Wesnoth (a hexagonal combination of Fire Emblem and Advance Wars with all kinds of mods for it).
Also, EA's released
Command and Conquer 1, Tiberian Sun and Red Alert for free, which is pretty awesome too.
We are loved even though we suck.
Psalms 37:37 (NHEB)
Mark the perfect man, and see the upright, for there is a future for the man of peace.