Using the MathJax context menu, you can access the source of any mathematical expression either in MathML format, or in TeX if its source format was TeX.
To access the context menu, right-click on a math formula (if you are using Windows), or Control-click it (if you are using a Mac). In the sub-menu
Show Math as you can choose between
MathML Code and
TeX commands to get a pop-up that allows you to copy the math source into another application. Try it out on the equation below!
You can also watch a screencast of a MathJax equation being copied and pasted into a variety of applications below. Note that this screencast is based on MathJax v1.1 so the menu structure is a little different.