Ok here's a really nice new Tower Defense game called Warzone Tower Defense.
Very nice smooth graphics and animation, nice range of weapons and requires real strategy.
Also has a few TD features of note:
- you can place walls and adjust route of enemies -- i always like this feature, adds nice strategy element.
- you have some direct-application weapons -- i'm not a huge fan of this weapon style, which adds an arcade-like element to the game.
- multiple maps to choose from
from http://flash.plasticthinking.org/ 
