BREAKING NEWS!!! The newest version of Desktop Tower Defense is out. My afternoon (and yours) is shot.
Update: New features include new Fun modes (Trickle- 1 creep per second, Random creeps), new Challenge mode (15 towers max). I'm on the scene, more as I have it.
Update: One new tower: ink tower, which has a minimum and maximum range and one new creep, a dark creep which I don't yet know how to kill (it seems to repel a lot of different attacks). My initial impression is that a lot of the changes make the game more complex but not necessarily more fun to play. Much more research is clearly warranted.
Update: It's also got in-game advertising...the little "K"s on some of the creeps refer to kongregate.com, a sponsor of the game. Blech. (Or maybe it's good that you can shoot advertisements?)