If you haven't been there in a while, or never, you should check out Washington state's unclaimed modey website.
http://ucp.dor.wa.gov/ Type in your name, your families name, your friends names, your doctors names, your job, whatever and see who has money coming to them! If you work it right you might be able to even talk them into giving you a reward for finding their money. :)
Every time I search, at least 1 in 10 people I search for has money, and usually it is more like 3 in 10. The best way to search is by first initial and last name. I just did a search of 50 people and found 2 had money. You can also do businesses now, which is cool. Get a raise from your boss for finding your job is owed money! Try typing in Boeing and see how many uncashed checks are floating around out there for them. Another idea? Try typing in the name of your church. I found both my church and my wife's parent's church were owed money. You could probably start a whole ministry just typing in the word church and finding all those churches that are owed money and let them know! One of my friends is even listed with an unidentified safety deposit box, and my old pastor is listed as having shares of stock!
It could come from anywhere, a prize you won but couldn't be delivered, a bank account you forgot about, a bill you overpaid when you moved and never got the refund, a check lost in the mail, an inheritance that never caught up with you even.
Go! Do it now! You would be surprised...