Pixeljunk's Monsters Delux on PSP! It's kinda short, but fucking hell it's awesome! I've actually got to play it on Casual difficulty because Normal gets too tough once you get to the medium-difficulty stages!
A while back I was searching for an open source version where I could customize the graphics to make a Zombie Commandos themed Tower Defense! If I could get my hands on an HTML5/java-script version that would be AWESOME.
Skibi's Castle TD from Warcraft: Frozen Throne.
After each wave of creeps, you play different mini games (like Mario Party). You get a chance to earn extra gold on how well you do and thus, improve your builder and towers. It's been years since I played the map.