Wikisource: proofread book scans side by side with the text in a free online library
Proofreading on Wikisource, the free online library of the Wikimedia projects, is open to everyone: a scan of the original document is shown side by side with its text version, and the volunteer compares and corrects the text. The workflow is described in the Help:Proofread guide, and there is also a Proofread of the Month project. The help page sets no deadline or age limit.
Volunteering Free Online