King Sol is the best with I think 70 different games. Also try Solitaire Engine.
www.runicsoftware.com/
It has a script engine so if your good at programming you can make your own solitaire game. I am still deciding whether to by this game or not. The card graffics appear blurry on my Axim but the demo will not time out its just limited you can't change anything game wise. Its the only solitaire set that has Scorpion and thats one of my favorites.