I've been playing a lot of Desktop Tower Defense. It's a simple game, but very addictive. Your goal is trying to shoot all the "creeps" that are trying to get from one side of the screen to the other, and you do this by placing guns in what amounts to a maze-like configuration.
You are given all the time you want for the initial configuration, but once the game starts it doesn't stop! You spend most of your time frantically upgrading guns or placing new ones of different types. Some creeps are immune to certain guns.
It's fast and I'm addicted to it - check it out! It has gotten into a few game journalists' and reviewers' Game Of The Year lists as well.