Pixeljunk Monsters on the PSN is also a very good Tower Defense game.

I too love Tower Defense a lot, they are the best flash games on the net, I'm currently playing Onslaught at the moment.

Here's a great site for some Tower Defense games: http://www.towerdefence.net/?page=allgames

Onsluaght is by far the best of the flash games, you have to put up a strategy on where to put them and make combos and such.

You should really try it out.

Also, agreed TD games are the best flash games by far.

  thanks for the link,
I would have tried google, but I was too lazy

tower defence is great, it is just a shame that either they get too difficult at some point (so you can not get to the end unfortunately) or they really get too boring/are too short, when it is continuous for so long it really isnt fun anymore and when it's over in a flash you are left with the feeling that you wanted more. Most of the flash ones are free but some cost money and to pay 10$+ for a flash TD is something i will not likely do.

you can also find the games on NewGrounds (tons of good games on there), and in the game WarCraft 3.

NewGrounds has one of the better TD groupings out there.

My favorites? Bloons TD 2, Onslaught 2 (made it to wave 500!), and Xeno Tactic. The Last Stand, if considered a TD, is very awesome as well.

My girlfriend is addicted to Onslaught 2 as well.

I got to level 320 on Onslaught before I quite on easy, very fun.