This is pure awesome. Finally something has been done! This is akin to the mythbusters going on TV and doing science instead of just talking about how awesome science is.
Apologies for my little rant above.
As for the site itself, other than being awesome, it needs a few tweaks. There is no place to discuss the site itself and possible improvements to it. Also, I wish there was a feature to hide the result until after I vote.
See the Feedback tab floating on the right.
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.)