I have been testing this web-based platform for a while, think of a merge between Solid Works and TinkerCAD.
Free with certain limitations. Simple export to STL. It can import/export to/from several popular CAD formats.
Still in beta, they say, but I guess that's more PR than lack of stability or availability. Invites seem to be granted effortlessly.
oops, almost forgot: [
www.onshape.com]
They have a nice collection of videos to help you start working.
(I am in no way affiliated with OnShape, just a happy user)