[Sale over!] Gamefly had sales on all 2K games.

EDIT 3: The entire sale is now over!

Also, Gamefly's generic 20% off code for this month is GFDSEP20 and stacks with this offer.

Steam codes are available for several of the games covered by the offer, including:

The Darkness 2
Spec Ops: The Line
NBA2K13
Civ V & Civ V Gold
Duke Nukem Forever
X-Com: Enemy Unknown
Borderlands GotY Edition
Borderlands 2
Bioshock Infinite

...so you can get two decent Steam games pretty cheaply.

Here's the link.

EDIT: If you're in the EU/ UK, you will apparently need to use this link instead, and the discount code: GFDSEP20UK

EDIT 2: The original sale offered Mafia 2 with any non-DLC purchase; that seems to be over now, so I edited this post to reflect it. 2K games are still discounted for now, though (even more steeply than before!) The old text is below for reference, but no longer applies.

They only sell Mafia 2 via Steam, and it says "receive your code" within 10 days, so I assume it's a Steam code. Non-DLC only, but you can get Mafia 2 as cheaply as a dollar if you buy the original X-Com (which is totally worth it, though I don't think you get a Steam code for X-Com itself.)