Gem Tower Defense by Peter Holko and Dorian Cox is, yes, another of the hundreds of flash tower defense games. But this one has a few new interesting mechanics: you place down five towers each round, but can only choose to keep one, the rest become rocks. You can also combine two of the five if they are the same level and color to make a stronger one. There are also special towers that you can create using special 'recipes' of other towers (the recipe list is shown under the game window).
I enjoyed it because of that mechanic, and because it also doesn't suffer from a common problem in similarly structured "mazing" tower defense games such as Desktop Tower Defense -- juggling, a cheap technique where you open and close routes to keep creeps wandering perpetually, which assures total victory if you know how to do it. It avoids it by not letting the player build during the battle period.
One big problem with the game is a lack of sound or music of any kind, but I assume the game is still being worked on. Recommended if you like the tower defense genre, ignore it if you don't.