Tower defense games have been around for a long time. The basic idea is that there is a never-ending wave of enemies approaching you, and you must put down defenses to stop them before they reach your base.
Fieldrunners for the iPhone is a particularly good one. Not only is it simple and fun, but it works well using the iPhone’s touch interface. I was surprised when it got me hooked. I played it obsessively for a week, which is much more than I can say for most iPhone games I’ve downloaded.
You can read more and view it at the iTunes Apple Store.