I have recently begun proofreading for Distributed Proofreaders, an affiliate of Project Gutenberg. By comparing the original text to the computer's adaptation, I eliminate errors. I also put the text into a standardized format to create a clean, polished look.
Project Gutenberg endeavors to make uncopyrighted works of literature available online for free. So far, they have posted 150,000 unique titles online, with works as varied as "The Tempest" and "The Iliad."
Proofreaders can proof as many or as few pages as they like. If you're only able to proof one page a day or even one page a week, you are still making a difference.
So log on to Distributed Proofreaders today, and start making a difference!