Prompted by Geoff in comments to this question, I thought of creating a poll of sorts where we can propose macros/extensions that should be automatically loaded for our MathJax installation here. Unfortunately I don't know where to get a list of the macros that are supported but not autoloaded by the beta MathJax, so maybe some kind soul could edit this question to include them.
The idea here is to post one answer for every macro/extension wished for, and then let the net amount of votes (up minus down) decide if autoloading should be considered by Geoff and the other devs.
Edit (W.W.): Since there are only seven sets of macros that are not automatically loaded, I've gone ahead and posted a stub for each of them (except for the chemistry specific one and
cancel which J.M. already mentioned). See Davide Cervone's answer for what commands are offered by each of the macros, and links to the relevant MathJax documentation.