EDIT: This promotion is now over.
What a funny coincidence. The day after the Billy's Sonic Site april fools prank, Sonic CD is free for android on the amazon app store. This is a really fun game, if you like 2D platformers. A lot of Sonic fans say CD is their favorite in the whole series. For me, its in second place. This version has a Sonic 2 styled spindash, both soundtracks (US and JPN/EU), and you can even play as Tails after beating it. I highly recommend it. Again, this is the android version.
Go get it here: http://www.amazon.com/gp/product/B008K9UZY4/ref=mas_fad_img