Desktop Tower Defense, designed by Paul Preece, is one of the brightest jewels in the fistful of free games that make up the Flash-powered land of the burgeoning tower defense category (think RTS lite here). While not the first in the field, it ranks among the best, with millions of games having been played since its introduction earlier this year.
In DTD, as with any tower defense game, automated opponents called creeps try to make it from one side of the play area to the other. Here they take the form of crude shapes traversing your desk. It's your job to stop them, but you can't shoot them directly. You can only purchase, place, and upgrade a variety of towers that automatically attack any targets that come within their range.
DTD comes with a good variety of towers to add to your defenses, and there always seems to be a number of challenging decisions to make at any given time. The game comes in three different levels of difficulty, plus several challenge and "fun" modes, and a multiplayer version is in the works. You can play it in any browser running Flash, and you can spend minutes or (more likely) hours tinkering with it, trying to get your defenses just right.
Give it a shot. Have some fun. And if you find yourself hooked on it hard, don't say I didn't warn you. Still, you won't have to look hard to find me. I'll be the guy chasing you up the leader board.