Beeminded successfully:

• Anki.
• Exercise, including pushups. I was also successful doing pushups with the Seinfeld method for about a year before Beeminder.

Beeminded with middling success or unsuccessfully (not totally sure why, but I think these are just harder goals for me):

• Hours worked (that I get paid for).
• GTD/Remember the Milk tasks.
• Hours spent on personal programming projects.

As far as pushups are concerned, they've been successful in the sense that I've been doing pushups regularly for two years and am still going, but -- due to my genes and my current choice not to expend a huge amount of mana points on this goal -- I've been plateaued at 12 to 16 consecutive pushups for a year or more. I second Daniel's recommendation [edit: Daniel being dreeves; also, jetm made the same point] to set a goal that's under the control of you, not of your body's ability to increase its strength.

I have lost money to Beeminder a few times. I think the money helps motivation, though maintaining the graph and "not breaking the chain" are powerful on their own.