i use an excel spreadsheet. strongly recommend printing and storing in a safe place in case of computer crash though... just in case the IRS needs that info for your next audit (sorry, i couldn't resist)
seriously though, i keep a spreadsheet to include when procured, from where, caliber, model, serial number, amount paid, approximate worth, and any additional notes (like an attached scope). one of these days i'll get more organized and take pictures and cross reference them.