~/documents/ is pretty common for games (Halo, KOTOR, NWN2, Oolite, Spore, ScummVM, SixtyForce).
~/library/application support is pretty common for productivity applications (Adobe, Mozilla, Microsoft, Handbrake, Skype, Vuze) and other games (Fallout, Porrasturvat, DEFCON, Myst V, American McGee's Alice, Prey)
I don't mind either way. ~/documents is easier to find and more often used by newer commercial games, and ~/library/application support doesn't get in the way of other documents.. I'm sure Apple recommends something, but I can't be sure what.
So take your pick
.
Best wishes,
Oscar
P.S. Still others put their save games in ~/library/preferences (Bioshock, Fable, Need for Speed), but that is the wrong place if you have more than config files to store..