Back when the forum started, folks found this program called Gun Safe, or something like that, which was a fairly simple database program, but was flexible enough.
Currently I keep a spreadsheet, make/model/year/distiguising features/ purchase price, serial number, dob (if known). I then try to routeinely update the values.
Prior to that I had a more complicated sheet that even tracked from whom purchased, or sold, and gains/losses per transaction.
During routine cleaning, I take pics of them all.
Spreadsheet is cool but I will start writing in a notebook.
Either email it to yourself securely, and not to a gmail account, or keep a flash drive and a burned dvd somewhere safe.
Most important use of the sheet.... know the serials in case they get stolen.... OR.... more likely.... god forbid when you pass away, make sure your next of kin or wifey/husband/partner knows about the list.... and knows what you actually have and what the value is.... so they don't trade it in at a gun buyback!.
Depending on the strength of your tinfoil hat.... Google Drive/Microsoft and others give you free cloud storage space for one reason... so they can learn about you. (for better advertising of course).