Tiny Heroes is a fun one from David Whatley, the mind behind the popular GeoDefense titles for the App Store (who's also, incidentally, speaking later on this week at the 360iDev conference going on here in Denver).
This one is tower defense as well, but it's got a nice Dungeon Keeper-esque twist on the genre. Instead of defending from the invading hordes, you must shut down heroes, attempting to invade your dungeon for loot. The look and feel is a lot of fun, and the interface is smooth and well-done, very reminiscent of Plants vs. Zombies (always a good thing). As you move through the game's levels, you get more and more tools to stop the little heroes, including fun things like mimic chests and spike traps.
There are over 50 levels in the game right now, with more coming later, and of course Game Center and OpenFeint integration extend replayability as well. Tiny Heroes is a lot of fun -- it's only available on the iPhone (right now) for US$2.99.