if you own half-life 1 you will have access to:
team fortress classic
counter strike
deathmatch classic
day of defeat
codename gordon (maybe a free addon for all?)
ricochet (maybe a free addon for all?)
you need to buy hl opposing force to play that game (standalone game)
you need to buy hl blue shift to play that game (standalone game)
counter strike condition zero is a seperate stand alone counter strike game, this gives you:
Counter-Strike: Condition Zero
Condition Zero Deleted Scenes
if you own HL2 you will get access to:
half-life source
counter strike source
hl2 deathmatch
day of defeat source is still on the coming soon list, dunno if it'll be free for hl2 owners when it's released.
so i think thats how it works. when i first loaded steam in, i put my hl2 key in (after installing from dvd). then my hl1 key, then opposing force key.
my valve folder is 6.78gb, thats without the standalone games of counterstrike condition zero and blue shift. so you need some space if you want to keep all the games on your computer.
also it's recommended to update steam. if hl source has come up just released you havent updated it in a long while (or never).