The Bookshare site, which provides free or low-cost ebooks to people who are blind or have significant physical disabilities, accepts scans from volunteers. Also, they could use people to proof books scanned by others.
http://www.bookshare.org/_/volunteer/overview
For book scans, they want books in RTF format and they have a couple of other requirements (like having original page numbers in the file), which may be a problem for books already scanned.
There are other services providing free ebooks to people who are blind but I'm not aware of any other that accepts outside scans. Also, Bookshare books are readable for people with disabilities who aren't blind, while books available from Open Library or the Library of Congress can only be heard using a reader supplied by a State Library for the Blind.