Our hosts at Tricycle Developments have created PredictionBook.com, which lets you make predictions and then track your calibration - see whether things you assigned a 70% probability happen 7 times out of 10.
The major challenge with a tool like this is (a) coming up with good short-term predictions to track (b) maintaining your will to keep on tracking yourself even if the results are discouraging, as they probably will be.
I think the main motivation to actually use it, would be rationalists challenging each other to put a prediction on the record and track the results - I'm going to try to remember to do this the next time Michael Vassar says "X%" and I assign a different probability. (Vassar would have won quite a few points for his superior predictions of Singularity Summit 2009 attendance - I was pessimistic, Vassar was accurate.)
Yes yes yes. Four times yes.
Right now the UI is so slow / bad that I couldn't see myself using it.
Agreed on the UI being incredibly confusing (and slow).
In terms of usability, if they just moved the judgment buttons down below, added text like "Render final judgment on this prediction" to make it obvious what judgment does, and changed "My 2 cents" to "Submit Estimate" or something like that, it would be a huge improvement over the current. These sorts of very minor cosmetic UI changes would be trivial to make.