Here's an interesting game I just bought. If you love the idea of a worldwide zombie epidemic, this game is for you!
You play on a map of the world with each player basically claiming a portion of the map (North America, for instance) to control.
At the beginning of the game there will be very few zombies on the map. But every turn they feed (which reduces the human population and increases the zombie population). Once they have reduced the human population in an area to zero, the zombies move to adjacent areas and start feeding again. You can build military units and develop various technologies that help you attack or defend.
But you need to be smart and move quickly, otherwise the zombies will overrun the map.
The game doesn't have specific solitaire rules, but it's essentially a "group solitaire" game to begin with (each player is essentially playing solitaire against the zombies in his or her section of the map to see who ends up with the highest score). So all you need to do to make it a true solitaire game is play by yourself and always try to beat your previous high score.
I think I'll get a lot of use out of this one.