I would say Incredibots, but it's 2D.
Decent game, but I think it's declining.
If you have the patience to learn how to script, Second Life's Sandbox area can be quite fun.
Sidenote: During my time on Second Life, I learned the basics of LSL and scripted a floating orb to follow people by modifying the already made yellow submarine script. I then scripted it to multiply upon touching someone. I placed a few of these down in the sandbox area and logged off for dinner. When I came back, there was an army of them terrorizing people. It was hilarious.
Sidesidenote: I've also made a magic amulet capable of shooting orbs of light, making forcecages, and shields. I had my auto-shield almost perfected when I got bored.
And yes, Second Life is free. You don't need to pay anything to fool around on Sandbox Island.
