I cannot stop playing Lux Touch for the iPhone. It's far and away the best game I've played yet on the device. While not quite perfect, it's an intensely addictive and fast-paced version of the board game classic Risk, but with a modern twist. And it's a free download.
You play the blue forces. Each turn, you get three armies, which you place in your territories. At certain times you can cash in "cards" (which you get as you conquer new regions), and those can be traded in for more armies. Get a full continent and get bonus armies each turn.
There's more to it, of course, and once you get the hang of it you can really have some fun. The UI is just hard enough. Sadly it's not adjustable, so if you get too good you might get bored. But don't worry, the developer of the game, Silly Soft, has said that it's planning a deluxe version before the end of the year with a much more crafty UI and several maps and modes of play.
If Lux Deluxe is even halfway as awesome as Lux Touch, I'll be first in line.