As much as this thread seems out of place here, this is a Frequently Asked Question, so perhaps if the thread's title was edited this would fit in well here.
Off the top of my head, the Big Idea Group,
http://www.bigideagroup.net, is one company that works with inventors on a "pro bono" back-end basis.
Evergreen IP is another such company that people have said good things about within this forum - conversation can be found here -
viewtopic.php?t=348Realize however, that companies like this as well as private investors tend to be very selective. Inventors who say they have a great invention that is unlike anything else in existence, and who are looking for "pro bono" work to help them license their invention - they come a dime a dozen. And often, their inventions are not easily marketable and/or unique, despite what they say or believe. So, even if yours is very marketable and very unique, its up to you to set yourself apart from the pack.