For proofreading experience, please try helping Project Gutenberg
Project Gutenberg ( http://www.gutenberg.org ) is an NPO effort that has been making copyright-free copies of old books available for many years.
Volunteers can help PG produce ebooks by proof-reading just one page a day.
For historical research, this project has been a great help, making many rare and valuable old books freely available online.
PS
Why would anyone want to be a proofreading volunteer?
It can be a painful and tedious occupation, especially for authors attempting to proofread their own work.
For others, for those who have a personal inclination for the work, it can be as satisfying as the proverbial nitpicking. As a metaphor, 'nitpicking' has a bad connotation, but in the majority world where nits still exist, it has an important social and hygiene function, and is something that is often carried out in a public, sociable manner.
Picking nits from one's own hair is practically impossible, and effective proofreading of our own writing is also difficult, though not impossible.
For those with the inclination to be nitpickers, proofreading can be satisfying... each problem discovered and fixed is a practical and measurable achievment.
For an author, seeing his or her own work cleaned up can also be a buzz, even if it reveals failings on the author's part. Being able to learn from failure is one of the main job requirements for most authors.
Authors can also learn by proofreading the work of other people. It is one way to gain confidence in writing, because the fear of making mistakes becomes less when we see the mistakes that other people make, including successful authors.
For someone wishing to become a professional proofreader, experience with Project Gutenberg could be useful as training, or it could be satisfying in itself, as a voluntary public service.