Since my kids take over most of the fun stuff in my house (TV, computer, Wii...), my iPhone has become my only backup device for gaming. At first most of the iPhone game were rehashes or hacks of stuff you can find online for free, but then I found Fieldrunners from Subatomic Studios.
Basically you have to stop wave after wave of enemy soliders and vehicles from getting into you fort. To stop them, you have 4 types of towers (gun, goo, missile and electricity) you must place in their path. The game play is not original, but the design is fantastic and completely addictive. I've beat the first map, but the second one on difficulty "hard" is really, really difficult. Definitely worth the money.