These games weren't always freeware, but thankfully the developers were kind enough to make them so.
Command and conquer, red alert, and tiberian sun
Grand theft auto 1 + 2
Team fortress 2
Now onto games that have always been freeware to my knowlage.
M.u.g.e.n - Funny game that allows you to import literally any fighter from any other game... wanna have barney the dinosaur fight mario, yeah this can do it for you.
Breakout clones - I like breakout... :I
League of legends - tis quiet fun but beware elitists
Trackmania Nations: Forever - Can be a rather fun arcade racing game.
Hedgewars - pretty much a worms ripoff, but it's free so hey
Quake Live - online quake for free baby!
Desktop tower defense - I love tower defense games.
and of course the best freeware game of them all...
MINESWEEPER!