MathJax is the best thing to happen to the web since the web began. I’ve said this a lot of times.
Sadly, MathJax isn’t loaded on every page. To fix this appalling situation, add the MathJax bookmarklet to your browser.
Update 21/05/2012: Jakub Kozisek has started maintaining a much better version of the bookmarklet on Github, so I’ll just link to that now: http://dzejkej.github.com/mathjax-bookmarklet/
Update 30/01/2013: Google+ uses some of the same CSS class names as MathJax, which made maths look all mangled when you run the bookmarklet there. I’ve updated the bookmarklet to override Google+’s CSS. For now, I’ve put the fixed version up here so you can install it, and I’ve started pull request on Jakub’s Github repository so he can merge the fix in.
Bloggers: if you’re looking for instructions on how to use MathJax on your blog, I’ve written another post with very detailed instructions.