Magic Defenders isn't new, but it has claimed a lot of my iPhone gaming time lately. It's a tower defense game, so if you like that genre, odds are that you'll like this one.
Instead of building towers, you play the game as a mage that's casting spells against waves of attackers. The game cycle has you casting spells using mana, killing attackers to earn XP, and then using that XP to upgrade your spells to kill attackers more quickly, earning more XP, and so on. It's a lot of fun and considering that the game has a few different heroes to play through and level up across three invasion episodes, there's plenty of content to explore. Blazing down enemies is very rewarding, as is learning how the various spells work together. You can slow enemies down with one spell, for example, freeze them in place with a second, and then nuke them all down with a third.
The game's great on the iPad, too, but the drag-and-drop spell interface makes it easy on any iOS device. And at 99 cents on the App Store for a universal version, Magic Defenders is one of the best bargains out there that you might have missed the first time around. If you like tower defense games or ridiculously addictive game cycles, it's definitely worth the buck.