Adobe has a new service called Adobe Buzzword at the new Acrobat.com website (free registration required). The service allows you to write and publish your own ebook in a format that is compatible with most ebook readers (including the Kindle, Nook, and Sony eBook Reader). Adobe Buzzword is an online word processor that you can use to create and edit documents in any web browser. The service supports collaborative editing so multiple authors can work simultaneously on the same document from their respective location.
Buzzword allows you to write an eBook on any browser and then export documents directly as EPUB files, a format that is supported by all popular ebook readers - including apps like Stanza that let you read ebook on a mobile device. You just compose your document inside Buzzword, export it as an ePUB file and then import it in your ebook reader.