yLend

From Spacejock Software:

If you've ever lent something only to lose it forever (A book, a DVD, tools from the garage and so on) then yLend is for you. You can add a loan in seconds: enter name, item and date and the loan is permanently recorded. When the item comes back, delete the entry!

In the interests of harmony with family and neighbours, I suggest you enter any loans AFTER they've borrowed your stuff and departed. Writing down dates and serial numbers in front of them isn't all that polite :-) On the other hand, if you're lending to a hoarder who never returns anything, feel free to make them wait while you enter the details into your computer right in front of them.