While we're pushing Tower Defense games, here's my current favorite, Gemcraft Chapter 0:
GemCraft chapter 0 | Armor Games. This is (oddly) the sequel to Gemcraft Chapter 1 which can be found here:
GemCraft | Armor Games
It's tower defense with a (small) storyline and an RPG-ish skill/talent/level system that makes you more powerful the more you play. I'd recommend starting with the first and then moving on to the second, as the second is a dramatically more complicated game. I've sunk endless hours into the game, so hopefully someone else will find it entertaining as well.