MathJax is a very useful tool to express many mathematical expressions. It is a fundamental part of asking or answering questions. Sometimes, I would like to visualize things to make them more explicit when asking or answering questions. However, I don't know an easy tool, so I don't usually bother with drawing.
Is there an equivalent tool of MathJax when drawing lines, circles, 3D objects etc.? If not, what do you do?