I've spent a lot of time looking an have not found one. I've found a number of vendors who sell this service, but nothing free. The SSA website has a link for employers to verify SSN's but they limit it to a small number per EIN and ITIN's are not included.
We use ChexSystems for all new accounts and they do verify SSN's as long as they're in the database; however we've found the database to be 3-6 months out of date. That's a problem when you're dealing with a lot of migrant workers like we do. An there's still the problem with ITIN's.
If someone knows of a cost-effective solution, I'd love to hear about it too!
If you approach life with pure logic you can avoid almost all of the fun.