Beeminder already has Inbox goals, and I have friends with goals like "Send X emails of type Y" per day. Currently, these involve manually reporting to beeminder whenever an applicable e-mail has been sent, or the size of one's inbox when the beeminder reminder arrives.

It would be lovely for there to be a Gmail plug-in (which might actually be implemented as a Firefox plug-in), which allows automatic reporting of inbox size (for inbox goals), and presents a "this email relates to goal X" option when sending e-mail.

I seem to recall Google has published an API for client-side plug-in writers (because RememebrTheMilk uses it), but I can't recall where that is off the top of my head.