I don't have a Kindle or Nook yet, and I'm not convinced I'm going to get one. I did, however, get this free Kindle application for my laptop, and also this free Nook app. Barnes & Noble also offers a LendMe option that allows users to lend books from their collections for two weeks.
Amazon and Barnes & Noble both offer a selection of free classics, as well as a rotating stock of modern works. Project Gutenberg is the most well-known of non-commercial free ebook resources.
Google ebookstore offers an on-the-spot reading experience instead of a download, but requires Internet, which is not always available in the wilds. I just added A Tale of Two Cities (free) to my booklist (it was the best of choices, it was the worst of choices), and Google held onto it for me.
How do you manage your reading in tight quarters?