Of particular interest to us Mac users:
- Civilization V / Civilization V Game of the Year Edition) (and most of the DLC)
- Civilization IV: The Complete Edition
- Duke Nukem Forever*
- Crusader Kings II
- The Darkness II
- Grand Theft Auto: San Andreas
There are also some bundles and great offers on Windows games too.
Important: All of these are the Steam versions (and include Steam keys for activation). For some reason, most of the Mac downloads (which are also for the Steam version) are not as heavily discounted, even though they are effectively for the same key. So get the Windows versions, activate the keys on Steam, and you’ll have both Windows and Mac versions in your Steam library (you can verify this on the Steam Store pages for each item).
* Not recommended, unless you really like that sort of thing
Strictly speaking, these offers only apply to people with a US billing address, but you can provide a US billing address and still download the games and get the code/downloads even outside of the US.