CreepTD is the new Version from CreepSmash and its a multiplayer TowerDefence game where you play all vs all or in Teams. Its free to play and written in Java. To play you need an account, but its easy to setup.
How does it work?
You have to do two things, first is to set up towers to defend yourself, second is to send creeps to other players and try to get 20 creeps through the defence. Every time you send a creep you also increase your income by 10% of the creeps cost. So the main target is to send as much creeps to increase your income and only build as much towers as u need to survive.
You will find the game at http://www.creeptd.com.