Runs locally or remotely
It’s still early in development, but it can do your basic primitives and addition/subtraction operations. (And of course export to STL for 3D printing.)
I’ve only played with it for a little bit, but I like the way that the UI makes it easy to make precision changes to positions and dimensions:
Derek Quenneville is a 3D printing evangelist who posts weekly on the Ponoko blog. Follow him on Twitter @techknight.