I started out w/ the firearms and then it got sillier and sillier. Pictures, purchase info, along w/ serial numbers, etc.
Most people don't keep serial numbers of anything they own, including VINs on their rolling stock so all that was added. Do you know the license numbers on your vehicles w/out looking for the registration? See?
Next the TVs and other serial numbered equipment. Stuff without serial numbers was inventoried by make, model, cost, purchase date, etc.
Sometime later along came some new furniture. Out came the digital camera again. I went ahead and made pdf files of the sales receipts of the new stuff. The pool equipment has serial numbers and model numbers. Everything new that comes in gets the treatment-pictures and copies of the paperwork.
To make the long story over, everything that I might have to prove that I own is listed. The insurance agent has a picture/number history. Call your insurance person and ask what happens if your house burns down. If he/she says something like "don't worry about it, we'll just write you a big check" then I'd start on an inventory. The agent tells one story and the adjuster will tell you a very different story!
Each of the boys has the spreadsheet, as does the insurance agent. Another is in my safe and another is in a safe at work. CDs are cheap and really easy to make. Make up a new group, write the date on the CD and spread them out.
Pain in the butt? You bet! Sleep better at night? You bet!