Addicting Internet Timewaster games about building guard towers on your desktop to stop little creeps. Desktop Tower Defense. Wiki.
Here are all the free Tower Defense Games. Lots of other games also there. This Enemy Territory is popular.
Top 5 Tower Defense Games from Just Whatever.
Originally designed as an expansion pack for Return to Castle Wolfenstein, Enemy Territory is one of the highest quality multiplayer freeware games available. There are serveral classes to choose from including covert ops, medic, soldier and field ops. There are always servers up to play on. Dont miss out!
This was from SF editor Nielsen Hayden at Making Light, who has more on tower defense games.