I too would recommend Excel. For the past 5 years I have used Excel as my checking register. This year I added an "Inventory" tab for all serial numbered property I own, be it electronics or firearms. This has really simplified things for me and also allowed me to use some of the forumla functions of Excel to track my inventory's value. After saving any changes, I frequently back it up to my MobileMe iDisk which (1) backs up the information and (2) provides access if needed through my iPhone or iPad. Every once in a while I also back it up to my iPod Shuffle for some extra redundancy.
Check out something like Dropbox for backing up offsite.
With my luck someone's already suggested it in this thread :/