my insurance company has a list of what camera gear I've got, and we file away documents & receipts in a filing cabinet. Turns out recently when we had our TV stolen the serial number wasn't on the receipt though so not much use there!. After getting a new TV and unpacking it, dispatched the box to the storage shed where - you guessed it, the old box was, complete with serial number so at least it's now recorded in the police records as being stolen.
Have since then updated a spreadsheet of serial numbers and e-mailed it to myself so it's accessable via webmail in necessary.
some fairly old canon camera stuff, canon lenses, Manfrotto "thingy", and an M5, also an M6 that has had a 720nm filter bolted onto the sensor:
TF posting: here 