I have a spreadsheet with make, model, serial number and any random notes I felt were relevant at the time. One copy on my computer and one on a flash drive. I figured if any of them were stolen, it would come in handy. The extra copy is because I assumed if any guns were stolen, my laptop would disappear as well.
Truth be told, I was bored as well. Seemed like a good excuse to spend some time and handle my irons as I have had almost no time to shoot since wifey and I moved to a new town.
Insofar as receipts, private purchases are legal in Texas so I don't have one for some of them. Particularly the older, harder to replace ones.