http://www.creeptd.com/

It's a multiplayer tower defense with multiple modes and it's cross-platform! You can read more about it here.

What's cool is that you can develop maps to be added to the game (Reminded me the process of designing Starcraft terrains). Link



CreepTD playerlist (Username -- SEN alias):

• Apos -- Apos
  
• You?
  

Dunno if mother of all TD games is the appropriate phrase for it. Gemcraft took that cake a long ass time ago. But it looks like something I'd have a look at.