BooksForABuck (prices range from $1 to $3.99); also has free books & links to other ebook resources. Multiple formats.
Double Dragon publishing; multi-genre publishers; different range of formats for different books.
Comics.Drivethroughstuff.com: PDF comics, RPGs, some novels. A part of the drivethrough collective; they own several domain names with different graphics but access to the same content. (Some content is only visible through specific search on some of the sites.) Some of the PDFs are locked & have social DRM (your name & account number in tiny text on each page); some aren't DRM'd at all. Good prices; quality products. Descriptions include page count & filesize.
Mobileread Wiki's got a section for
non-DRM ebook stores.
Not going to list adult/erotica publishers. There are many that sell non-DRM'd ebooks. Many feed their titles through Fictionwise, although some only send selected titles to FW. (If people are actually listed, I'm willing to put together a list of erotica/adult ebooksellers, but I figure it'd mostly be a distraction from the main thread.)