Your lawyer, your insurance company, your executor, and your family may love you for this Home Inventory Guide one day, though only one of the latter is likely to help you complete this guide:
Via Neat New Tricks (which also links to this Picanswers site – gotta love the web), comes this: Home Inventory Guide
It’s a California guide so don’t phone them for help, unless you live in California! Phone your own state’s “people.” (You DO have People, don’t you? If you don’t know who they are, phone your public library reference staff – they are Your People.)
Completing this guide is way more work than most people could possibly imagine ever doing (except for those of you not putting off cleaning out your closets until after you retire), but it IS possible, even if the goal is to complete this one page a week, you would still get it done at a reasonable clip (compared to never doing it at all!).