Smashwords.com has self-published ebooks, including plenty of romance & erotica titles at good prices (I'm currently enjoying Patricia Ryan & Alexis Harrington books at $3 each), and all DRM-free. Quality is hit-or-miss, but they offer partial downloads, often 50% of the book, so you can sample & decide if you like it.
Fictionwise.com has more mainstream publications, including both DRM & non-DRM ebooks; their non-DRM'd are available in several formats.
Feedbooks has free ebooks, including public domain, author releases, and creative commons releases.
Many of the publishers who list books at Fictionwise & Smashwords have their own websites, sometimes with more books that aren't available elsewhere.