Today I signed up for Rebate Tracker. No idea why I didn't do it years ago.
They have most rebates already in their system so you don't have to keep track of all of the data before you send off your rebate. You enter the details about when and how you mailed it and voila, everything is stored for you. I made sure to enter the phone number for the offer since that was the only detail missing. I'm sure an Excel sheet would do the same job, but I kinda like gadgets - especially the free ones.