You may also be interested in our toolset (GNU-compatible only at the moment, we're working on converting everything to Python but we're not there yet): https://github.com/standardebooks/tools
I'm happy to answer any questions anyone has. We're also more than happy to have new contributors, if you're interested in working on and proofreading a public domain ebook that you've been meaning to get to.
Some of you have mentioned concerns about the modernizations we do. The key word I think is "light modernization". Mostly that just means bringing spelling up to modern standards, and removing a lot of hyphens in words that are no longer hyphenated. A common one, for example, is to-morrow -> tomorrow. Another one we recently added was lacquey -> lackey. Generally we leave punctuation and grammar alone. I liken this to modern books replacing the "long s" character--it's just presentation that doesn't affect the meaning. Modern readers would rather see "successful" instead "ſucceſsful" even though the latter is what was originally printed.
I struggled for a long time with my desire to see older books with modern spelling and typography, versus preserving the intent of the author and original publishers. Over time I've come to realize two things:
1. Many books back in the day were heavily edited by the printer and publisher without the author's input anyway, so you'll get various editions over time that look totally different. Jane Austen books are a good example of this--early editions often have a pathological overuse of commas, while later editions published after her death just remove a lot of them without comment. So when we're producing our own ebooks, we accept that there's a level of editorial discretion involved, and that "the author's intent" was a very fuzzy and often totally ignored topic hundreds of years ago anyway. How can we tell what the author's intent was in the first place, if various printers and publishers have meddled with the editions for hundreds of years already?
2. For those of you who want to read the originals in their totally unedited form, other projects like Project Gutenberg or Wikisource already have those faithful transcriptions for you, and places like Internet Archive, Hathi Trust, and Google Books have the page scans for you. By lightly modernizing our own productions, we in no way diminish your access to the painstakingly-preserved digital editions; we're just adding another option for you to read.