The newest free game as part of Xbox LIVE's 'Games with Gold' is Rainbow Six Vegas. I know what y'all are thinking: "Rainbow Six Vegas? Isn't that game, like, a billion years old? And wasn't there a second one? Why aren't we getting that one for free? Or both?" Well, I cannot answer any of those questions--though, I can confirm Rainbow Six Vegas is only four years old and not a billion years old--but, if you happened to miss out on the Rainbow Six Vegas games, both are great and, despite being old(ish) at this point, Rainbow Six Vegas holds up extremely well and is full of dozens and dozens of hours of enjoyment--'Terrorist Hunt' mode is still lots of fun.
P.S. Grab some free Rainbow Six Vegas DLC while you're at it. (It's always been free.)
Rainbow Six Vegas [Games on Demand] - ($19.99 ---> FREE)
- Rainbow Six Vegas - "Player's Pack: Red Edition" [DLC] - (FREE)
- Rainbow Six Vegas - "Player's Pack: Black Edition" [DLC] - (FREE)
And remember, the Xbox LIVE 'Games with Gold' promotion is only open to Gold members (thus the name), so if you're rocking a Silver (Free) account, you're shit out of luck.