Update: As promised, watery GTA game Assassin's Creed IV: Black Flag is free until the 18th of December. Just pop over to this page, sign up with your Uplay account (you have one, right?) and presto! Free Assassin's Creed game that is not really about Assassin's Creed at all.
It is hella fun, though, so check it out.
From now until December 11th, Ubisoft is giving away 2007's RTS World in Conflict. After that, Assassin's Creed IV: Black Flag will also be free until the 18th of December.
The games come with no strings attached, requiring one to simply log into their Uplay account and allowing one to keep them forever.
If you would like to know more about them, World in Conflict takes place in World War III (that's 3) after an alternate 1989 caused the Soviet Union to invade Western Europe. Black Flag, meanwhile, is a fantastic pirate simulator whose only connection to Assassin's Creed is in the title -- yours truly likes to refer to it as "GTA, but in the seas".
Go here if you want your free game. We'll update this piece when Black Flag comes along for keeps, too.