Readers Ndex is a centralized site for publishers and booksellers. Small publishers are featured and the site provides the usual interviews, sample chapters and search engine. The Tattered Cover handles book orders but there's a free service that allows any bookseller to link to the site for ordering. http://www.ReadersNdex.com/.
The Internet Book Fair provides homepages and catalogues for a number of small presses. There's a directory of booksellers, as well as information and links to book-related sites around the Web. http://www.bookfair.com.