ImplicitCAD is a free open-source 2D and 3D design tool maintained by Julia, and now it’s available as a web-based service. It’s text-based and parametric. Head on over to http://implicit.faikvm.com/ to see a demo and design your own parts and objects, then download them as STL and 3D print them. For example, this is a child-like artistic impression of a bolt that would not work very well at all (feel free to click the link and improve it though):
# Tips: Yes the connection is slow right now, it’ll be improved shortly. Click Render and wait a minute.