Calibre (Free: Open-Sourced, v0.5.12) is a app that attempts to be the iTunes of ebooks, which such features as conversion, library management, and downloading to devices.

It supports converting numerous formats to various ebook formats easily, and has a built-in reader for the ebooks as well. It will also use the feeds from popular magazines and news sources to generate ebooks that can be read on the go. Finally, it supports downloading ebooks to Stanza the popular ebook reader for the iPhone. These features make it a worthwhile download.