Symphonic Tower Defense is a rather quirky and interesting tower defense game.
Specifically, it's a heavily music-themed game wherein the enemy waves' composition and aspects of your towers are based on the music that's playing on a stage.
This is a really interesting concept, and I enjoy playing it, although the game as it exists is perhaps too easy up until the last two or three levels, where there's something of a difficulty spike. (If you invest in the right stuff, there's more or less no way you can lose. If you use "Decrescendo" and "Repeat," which are essentially traps on the paths, it radically slows down your enemies; cluster some good towers around those parts of the paths and it'll be pretty rare for anything to get past.)
In fact, while there are plenty of other tower defense games I like better, I have to say that probably the only flaw with it as presented is that you can't plug songs other than the provided ones into it. If somebody got on that, we'd have a game that would have more playable "stages" than one would ever need.
Anyway, interesting concept, fun to play a few times, and it might introduce you to some neat songs (the game links to artists' pages for download and junk). What's not to like?