I've been hooked on Desktop Tower Defense for a couple weeks now. I couldn't give you a good reason for it, it's just fun. Simple browser based flash game with very simple rules and a really growing community.
Here's the blog post that finally got me to admit my addiction *grins*. As many authors have said, if you have the right personality you are going to want to avoid this game. It gets very addicting trying to survive the final few waves.
You've been warned! lol