Shapesmith is parametric, so it's very quick and easy to modify your models. Each model is defined in a hierarchy of objects, and you can edit any of the objects in the tree at any point. The whole model will re-evaluate automatically
You can even use variables with expressions for dimensions like widths and heights
Export your model to STL and print it on your 3D printer or send it to a service.
Even though Shapesmith is a web application, it is also open source. 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.