Tower Defense games

Dungeon Defenders 1 / 2

Orcs Must Die

Defense Grid

Super Sanctum TD

Sanctum 1 / 2

Defender's Quest

I like the Sanctum series.
Specially Super Sanctum TD.

Deathtrap maybe? It's different from your usual tower defense games though.

Also, I should probably point out that I don't own the game. I just assume it's good because it's based around the tower defense mechanic from The Incredible Adventures of Van Helsing, which I liked quite bit.

Sanctum, Tower Wars, Prime World : Defenders, Anomaly

Anomaly series (reverse tower defense)
Defense Grid 1 and 2
Orcs Must Die 1 and 2 (third person shooter tower defense)

Those are on the top of my list of ones I like.

Defend Your Life it's an anatomic based tower defense

First of all, I would like to say that I am really not a big fan of Tower Defense games.
That being said, I would recommend Winged Sakura: Mindy's Arc.
Super fast paced, RPG elements, and great art.

Bardbarian
Commando Jack
The Culling of the Cows
Go Home Dinosaurs!
PixelJunk Monsters Ultimate
Yet Another Zombie Defense

Sang-Froid - Tales of Werewolves is quite interesting. Not a pure tower defense, but that element is there.

Sol Survivor

I'd recommend others but really all you need is Sol Survivor.