I have stumbled across a few new tower defense games that I found worth while.
Astral Tower Defense is a nice laid back TD that was inspired by a space MMOG called Pardus. Although I haven't played Pardus before I have played a couple maps in Astral TD and I am pleased with the game.
It has plenty of levels and the monsters produce plenty of loot for multiple strategies resulting in a high replay value.
Hands Of War is another game published on Armor Games. I liked this game because of the RPG feel that it developed. I also enjoyed controlling a hero and being able to teleport him across the paths. The idea of upgrading your heroes stats and your towers stats were wonderfully intertwined. This game is definitely worth a try.
Horde Of Hordes is brought to you by Armor Games as well. It is a fun game that has no set path for the monsters to follow. What I liked about this game was its simplicity. I thought it was some time well wasted.