A new alternative to web-based 3D modeling tools is emerging: Shapesmith. We say emerging because it's an unfinished work. Shapesmith is an open source project that offers anyone the chance of running their own 3D modeling tool from a website.
The interface is truly beautiful and aesthetically simplistic, but as such it has some limitations when you want to develop more complex models. You must draw primitive objects such as cubes and cones on the baseplate, then move them about. Shapesmith offers the basic boolean operations including union and subtraction so you have a chance to produce interesting designs. Finally you can export your models to STL format for 3D printing.
Just don't accidentally hit the "back" button like we did.
We think this is a great start and wonder how quickly it will be refined into a usable tool. We might also see someone make a commercial version of it to compete with Tinkercad and 123D.