Mine's on an Excel spreadsheet with different tabs for different items (Carded figs, Loose, Vehicles, Lego) with columns for quantity, if the item is trade fodder (carded figs only) and comments, which is no where near being updated. Once I get a new item, I update the list.
I still have to do a tab for all the misc stuff I have (mugs, glasses, Hallmark, etc) but I do have most of it written down. It just needs to be put on computer.
And James, if you're interested in my template, send me an email at firstname.lastname@example.org
and I'll forward it to you.