The GameTap lite client has been out on the Mac for a while, but GameTap just finally released a new version of their full player, and it is now fully compatible with the Mac.
Unfortunately, things aren't quite hunky dory just yet-- the Mac player will only play Mac-supported games, which leaves a large number of games completely out-- Sam and Max, Psychonauts, and any other games listed as "Windows" are N/A in the Mac player. However, any emulation titles are in, so Genesis, Neo Geo, and the old arcade games are all playable, according to GameTap's page-- you can get your Sonic on in OS X.
This still doesn't help us with new games, of course, but it is awesome to have a huge library like that now available for gaming on the Mac. The GameTap player is now available for download over on their homepage, and while many games are available for free, a Gold subscription to play all the games is $60 a year.