They say nothing in life is free. That may be true but the following links to literally dozens of free games ranging from 3D shoot-em-up (Postal 2 mp, Wolfenstein ET) to turn based (Scorched Earth 3D) come pretty close!
Many of them are crappy but there are some older classics that later became free.
http://www.hardforum.com/showthread.php?t=1389026
