Between now and the 15th, you can nab Crackdown, which is usually 800 Microsoft Points (that's $10 in non MS-moon money), and after that till the 31st, you score both Dead Rising 2, normally $14.99.
MS is also includes that game's DLC prequel Dead Rising 2: Case Zero, usually 400MSP or $5, for free as well, which is nice. Though, as before, it would appear that Microsoft almost doesn't want anyone to find out.
If you log into your 360 dashboard, you won't find a big heads up like Sony normally does when offering free games for their (rather successful) PlayStation Plus initiative.
Instead you'll have to select the Games tab, then choose the Browse Games box, and then you'll see a tiny notification in the upper right hand corner. Also, the selection is not quite as fresh as one will find on PSN, though once again, free games are free games.