I use email a lot. I've been looking for a way to add notes to emails so that I could save my thoughts ("next action" in GTD-speak) when I come back to them. I use Thunderbird, so ideally what I want is an add-on for it.
I tried XNote, but found that on the Mac is was too unreliable - I constantly had problems with notes not appearing. (I did not try the experimental XNote Plus add-on.)
I found Notary, but it doesn't work for version 2 of Thunderbird. But then I stumbled across this unoffical port of Notary to Thunderbird version 2. I installed it and so far it seems to be working fine. It adds a panel at the bottom of each email into which I can type notes that persist even when I move notes from one folder to another.
I did find I had to disable my Nostagly add-on which added keyboard shortcuts, otherwise those shortcuts take effect when I type keys into the notes panel.