Allow for buddy or referee reported goals
Currently users self-report on beeminder goals. That's good, but we can lie, we can round things up/down, and we can take samples at convenient times. If there's money at stake, and that money is significant to the user, then some users will simply misreport rather than fail a goal.
However, if you have a buddy reporting a goal, you can't as easily weasel out of it.
This is a suggestion to allow nominate (on a per-goal basis) another user or users as "buddies" who can also enter data for that particular graph.

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Michael commented
Couldn't you achieve this by having your buddy sign up for Beeminder and have his goal be "Keep my buddy on track"? He just enters your credit card info so you get stung instead of him ;-)
Or you could just allow him to spoof your e-mail address and have him e-mail in data points on your behalf..?
It seems there are other ways to do this besides a code change..? Just dumping a few out there.