Evolution Defense is a pretty good tower defense title. While it doesn't add a whole lot to the genre (there is some innovation in terms of how towers are upgraded, and support towers can be built and added in around damaged towers), it is a smooth and direct TD game that's made for the iPhone.
Your job is to attack pathogens that are invading a bloodstream, and as with the usual tower defense spiel, you need to build up towers along the pathway (according to the currency that you have). It's pretty basic stuff, but a solid amount of tower choices and some excellent (if a little strange) graphical representations make for a quality game.
Best of all, it's completely free. It seems like it's been a while since we featured a freebie, so head on over to the App Store, and pick it up right away. Facebook integration will add a little extra oomph to the gameplay, and there's supposed to be a pretty good multiplayer mode (that, unfortunately, didn't work on my 1G iPhone) where you put your own creations up against a friend's tower setup. All in all, it's an excellent bit of fun for the low, low price of free.