Grand Old Games sells mainly classic PC games and some modern indies as DRM-free downloads. They're having their winter sale right now, with different games on sale each day for as much as 75% off. Most of the games are PC, some play on Mac or Linux.
They have a nice sprinkling of the true classics from the eighties and nineties, so many of the graphics will be dated to beyond dated, but there is some great storytelling and fond memories there, if you were a gamer of the era.
Some of today's bargains that might be remembered:
Wizardry 8, $2 regularly $10
The 7th Guest $1.50 regularly $6
Some other classics you might recognize, not on sale at the moment:
Baldurs Gate complete and BG2 complete, $10 each
Alpha Centauri $6
Master of Orion 1&2 $6
Wing Commander 3 $6
If you spend $5 during the sale you get a free copy of Divine Divinity, which reviewers compare to Diablo. Spending $50 gets you Blackguards, a fantasy strategy game.