This is a really great resource. I highly recommend checking it out.
Alternatively, archive.org is another, imo even better, place to find loads of interesting free ebooks as well as anything else in the public domain, be it music, movies, and so on. The epubs, in my experience, aren't as great as gutenberg's due to shoddy OCR software, but they have pdfs that are fine.
google books also has some neat public domain stuff I can't find on any other free ebook site, and if you spend enough time, you could compile together the entirety of the western canon here:
Which I know some people dislike due to issues with under-representation of minority/female authors, but I just treat it as a very large reading list of stuff worth reading and don't treat it as an end-all list of the only books you should read.
HOWEVER, my absolute favorite place to get books is from this little known site:
Which is also trying to compile the western canon, as well as any other interesting author (In the public domain of Australia), all in high quality epubs (Or mobi) free for all to download. The last time I downloaded the entire website, there were 2,565 epub ebooks. That was about a year ago, and since then the guy updates more or less every month, I'm sure there's at least a couple hundred or so more now.
I personally like this place the most because since they try to compile the great books and authors of the western world, you're guaranteed at least some standard of literary quality, and it's all in one place that is neatly sorted and very easy to download in bulk if you know your way around a bash shell.
As a tiny warning: if you're not from Australia, there might be some (not a lot) epubs there that aren't in the public domain in America and thus are illegal to download if you are a resident of the US... Just, you know, don't download them, and if you do download them, don't tell anyone!
Speaking of Australia, its gutenberg equivalent
is also a great place that has some books that aren't in the original project gutenberg due to copyrights and stuff. Same warnings apply.