In addition to the other suggestions, here are a few more. I'll leave it up to you to decide if these fit your particular set of rules for ethics and level of convenience,
Amazon's "Best Sellers" are worth browsing. There used to be an easy link to it, but you have to look a little harder now to find it. Or just bookmark
this link.
Once there, you can narrow by genre. First, check the free books. Many independent authors have started making some books free, either old titles or firsts in series. If you like to read a wide variety of authors and have a high tolerance for wading through crap, you may never run out of books. Incidentally, most indies lack DRM.
Checking the paid books is a good way to find sales. Reasonably popular books that go on sale tend to shoot right up in the listings. Browse the first few pages of paid best sellers in you favorite genres to find the best sales, even if you ultimately buy them from somewhere other than Amazon.
If you're into public domain books,
books.google.com is a treasure trove of stuff that hasn't even made it to Gutenberg. Whether you want some old
eugenics-based science fiction or
a recipe for rabbit soup that can be adapted to squirrel, Google's got you covered. Use the
advanced book search.
The old
Baen CDs are still available online and are still legal. Note that I've had trouble with some of the ISO images, but the "CD.zip" work fine for me.
While
Christianbook.com obviously has a somewhat narrow selection of books, they're one of the only bookstores that still makes it easy to specifically search for
ebooks that are on sale. Order by price to find the freebies. Note that Christianbook doesn't let you download the freebies, but they're almost always free at other stores (Kobo, Amazon) as well.
The University of Chicago Press has a
free ebook every month. The topics are varied and potluck, having included literary fiction, biography, tomato hybridization, sexy nuns, and a history of ambergris. Not all of the subjects have been to my taste, but many have and all are high quality. Unfortunately, they continue to use Adobe's ADE DRM.
Aarhus University in Denmark also has a
free monthly ebook (two, if you also
read Danish). They're only available as PDF and tend to be a bit more esoterically academic than University of Chicago's offering, but I've read and enjoyed enough that I find it worth a look. There's no DRM.