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. No install required - if you have a WebGL-capable browser you can use Shapesmith. Shapesmith is built with pure HTML5 and Javascript and makes use of various new web technologies.

Operating system: Any system with modern browser
Requirements: Node.JS, Browser with HTML5, JavaScript and WebGL capabilities