Smashwords, which distributes to iBookstore, Barnes & Noble, Amazon Kindle, and Kobo (among others!); Amazon Kindle 3; iPod Touch; Barnes & Noble Nook; Kobo Reader (and Borders); iBookstore; Vook; Enhanced Editions; Augen’s The Book; Pandigital Novel; ECTACO jetBook; Aluratek Libre; Cruz Reader; PC software for Barnes & Noble (which has hooked up with HP), Kindle, Kobo, and Sony Reader; iOS software for Nook, Kindle, and Kobo. And Aldiko must go in there too, which is important for Android devices. That’s off the top of my head. Probably other things not surfacing this early in the morning.
Pocketbook, Copia, Onyx, Hanvon, Jinke, Samsung, Audiovox, Netronix, and Gajah are also missing. And there are a number of open source projects that deserve a mention: calibre, Sigil. And don't forget Goodreads, Wattpad, BookGlutton, as well as the Pocketbook-Netronix merger.
This really should be updated, but it's beyond my graphics abilities. Does anyone want to take a shot at making a new one from scratch? I'll help.