Distributed Proofreaders: online volunteer proofreading of public domain books
Volunteers proofread pages of public domain books by comparing the scanned image with the OCR text so the books can become e-books. The work runs through several rounds of proofreading and formatting, and pre-released e-books also get a smooth reading check. Registering on the site and confirming an email address is all that is needed. There is no required workload: the site suggests proofreading at least a page a day, but volunteers choose their own pace.
Volunteering Free Online