Update: Annnnnnnnnnnd it's over. Sorry everyone who just noticed the post. Original below.
If you have heard about the On The House promotion from EA's Origin service, then this will sound familiar. For the next eighteen hours, the Humble Store is giving away Tropico 4 from Kalypso Media Digital. It does not include any of the expansions or DLC, but it normally retails for $17 CDN and you're getting it for free if you redeem it before it expires. Also, according to the Steam page, the game requires that users register a (free) account with Kalypso to login.
Again, you're getting it for free, though, and I know of a few people that love it. Tropico is a city-building game of sorts, except that, instead of playing as an omnificent being that benevolently rules over a region, you play as an actual dictator. I haven't played it myself, but I just added it to my Steam account, so I'll hopefully get around to it soon.