There's plenty of public domain stuff via Project Gutenberg
(main source of it all), and the Internet Archive
, and you can check the wiki for sources of free ebooks
, which many universities and specialty archival projects are putting up online.
Plus places like Manybooks
, not to mention our very own MobileRead members make nicely-formatted versions
for many kinds of reader, and all free because they're out of copyright.
There's also Girlebooks
which both re-formats classics written by women, and also reviews them at the same time, if you were looking for reading recommendations for perhaps more obscure stuff.
And if you're interested in science fiction/fantasy, the Baen Free Library
gives away the first book in many series as a promotion.
Most places have downloads available in multiple formats, and I've gotten a lot of good free reading this way.
Kindle and Nook are both "closed" in so far as the DRM copy-protection applied to any purchases you make from their respective e-book stores go. But the formats they can read are relatively open (Kindle's Mobipocket is not an open standard like ePub is, but many tools can convert to and from it easily enough).
Nook will support ePub out of the box. Kindle does prc/mobi. Both are easy to convert to and from each other as needed and are usually the most popular and readily available (after PDF and HTML) e-book formats available on most websites.
If you've a B&N store or a Target or Borders in your local area, perhaps you might want to visit each and check out their demo displays of nook, Kindle, and Kobo, respectively. Try out each reader, see if there's anything especially appealing/bothersome.