The best one I've found: Cult3d
Nokia's phone introduction is based on it. Works very fast under NN, Mozilla and IE - just 1 small plugin needed. You can find a few impressive presentations at
www.cult3d.com
The minus of it is the symbolical number of recoignised filetypes (there are only 3 exporters available: for Maya, Plasma and 3DS) and lack of 'third party' converters. Andy, maybe plugin for export, before other will put it into their modelers?
I've seen also Java3d in action (suspends my Opera) and jvue (lot of 3d models recoignised - including typhical CAD files, but slow, takes 300MB on HDD and works as 'client-server')