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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.)