Banks, insurance companies, etc are required to send dormant funds to the state of Texas after a certain amount of time so the state can use the funds or collect interest on it until it is claimed (if ever). Most other states probably do the same.
Texas does publish lists of unclaimed funds periodically & has an easy-to-navigate web site to search for such funds. The process to file a claim is also very simple & the state pricesses claims fairly quickly. I have no idea how well it works in other states but I give Texas high marks. My wife & I have recovered several "forgotten" funds over the years - nothing huge, but found money is always nice, even when it was already yours in the first place.
One word of caution - file your claims directly with the starte. There are many web sites who are glad to "help" you search for the money & file the forms. While this is legal, the value of their service is not really worth much.