I really enjoy playing defense grid on Steam and playing Tower Madness on the ipod touch. So are there other decent tower defense games out there that are worth looking into?
The Bloons Tower Defense games on www.ninjakiwi.com are the best. Onslaught is quite good as a very straightforward tower defense game. I would go to www.towerdefence.net and arrange them by highest rated. There is a game where you defend your pizza from ants that I am particularly fond of.
Pick up Starcraft 2. It has a thriving custom mapmaking community(from sc1), and is the originator of the tower defense games. It also has plenty of ‘last stand’ type games which are similar to TD, except that your units(usually terran) form a defensive line to ward off wave after wave of enemies(usually zerg).
Warcraft III is where I first became addicted to TD games. Online TD with Warcraft III is amazing.
I also recommend Plants vs Zombies. GREAT game- if a little easy.
Are there any good ones out that involve mazing? I’ve noticed lately that most of the TD games involve building off the enemy patrol path and simply killing things fast enough, the ones I liked back in Warcraft 3 involved building mazes around the points where the enemies changed direction to maximize the time around your towers.
Defense Grid (well, the demo) had some areas where you could sort of maze, but it wasn’t nearly as free form as I’d like.
If you like mazes, the later maps in Defense Grid: the Awakening require it. It just starts off easy, then gets harder fast. I like to think I’m not too bad as far as gaming goes and I had to re-do some of the later maps over a dozen times to win them. Best tower defense game I’ve ever played for sure. 
Two that I’ve played a lot haven’t been mentioned, both on Kongregate.
Desktop TD Pro allows mazing. If I recall, it was good to run the creeps past the ink shooter a lot.
I never fully completed Cursed Treasure: Don’t Touch My Gems. I liked the Dungeon Master type theme and replayed level 14 at least a dozen times, but the last “Brilliant” rating eluded me.
Ooh, damn you, fruitbat! I played an earlier version of Bloons a year or so ago, and got addicted; last night the cravings returned full-force.
Plants vs. Zombies is one of the greatest tower defence around.
I was deeply addicted to FieldRunners for about six months.