Second Life (SL) is a 3d virtual world (some would call it an online game, but it's not really that; for one thing, it kind of sucks as a game, and for another thing, it's much more than that). You can do modeling and building entirely online. People have created buildings, vehicles, sculptures, animals, hairdos, and any number of other things using SL's relatively easy to learn modeling tools.
Builds in SL are made out of "prims," short for primitives. There are a relatively small number of primitives; a block (cube), a sphere, a cylindar, a torus, a ring, that sort of thing. You can then torture them -- stretch them, shear them, twist them, etc. You can scale them, texture them, and put them together to make all kinds of stuff. This week, SL introduced something I"m very happy to see: "sculpted prims," or "scupties," a way that you can make a generic 32x32 mesh into a single prim. This allows geeks like me to use something like Blender to make a model and export it into something that tells SL how to shape its prim. To the right, I'm standing proudly regarding my first scuptlie. This is supposed to be a star filling its Roche lobe. I'll use this as I redo the model I've made of a nova and/or type Ia supernova progenitor, which I'd clumsily built out of prims in the past. This thing was easy: make a sphere in Blender, use the proportional edit tool, and pull out a lump on one side.
(The cat was not built by me, and has been around for quite some time. It is built entirely from traditional prims, no sculpties involved. It walks around my island and meows at me every so often.)