dirtyharry50: Steam
Mac App Store
Origin
Blizzard Store
Amazon.com
Humble
Groupees
I should be able to find enough games on those places including classics since Steam is picking up so many regularly now. They just recently added the first two Descent games for example.
Which is all well and good, but what about the issue of being DRM-free? Humble is the main leader here, but their record lately has been very spotty, what with Steam becoming more of presence there.
It's worth also pointing out DotEMU, which is currently now undergoing a massive 70% off sale for their 7th anniversary. I got Aliens vs. Predator Classic 2000 and two of their Sega Genesis bundles for almost nothing ($1.50 and $3.00, respectively). And yes, they work very well in WINE on my Mac. :)