It's called 2048 and I can't stop playing it.
2048 is deceptively simple: Using just the four arrow keys, you move numbers about on a small board.
You'll get the hang of it quickly -- bang two 2s into each other and they make a 4; two 4s make an 8, and so on.
It's a good brain practice for multiples of two. If you can get all the way up to 2048, you win.
Hence the title of the game. You can play it on the Web right here.
There's a Web version that's fun to play on your phone, too - you just swipe the numbers with your thumbs. it's at http://git.io/2048.