https://tinkercad.com is another one that I have been using lately. It seems to be focused on editing more, but the community and library are quite good as well.
I'm always amazed at how well WebGL works - I used to think that 3D intensive applications would be one thing that would/could not be converted from desktop app to web app.
I've been using tinkercad for a few weeks and I absolutely love it. The editor is amazing.
I'm not really sure I see the point of the original post though, since it is just a viewer and not an editor. tinkercad's ability to import, edit, and export stl is extremely compelling.
I'm always amazed at how well WebGL works - I used to think that 3D intensive applications would be one thing that would/could not be converted from desktop app to web app.