Shapesmith is parametric, so it's very quick and easy to modify your models. You can also use variables with expressions to quickly change models, like changing the radius of holes:
Import STL models and edit them, and export your models to STL and print it on your 3D printer:
Shapesmith is completely open source under a permissable license (MIT). You don't have to worry about the hosted version disappearing, you can always run your own server. You can also run you own server in your school or business to keep your models private.
Shapesmith is on Github.