The three readers are Adobe Digital Editions from Adobe, Stanza from http://www.lexcycle.com/, and Skim from http://skim-app.sourceforge.net.
Each of the three have their strong points and weak points. Adobe is fairly powerful and fast, but lack many of the whiz bang features of other readers.They concentrate mostly on PDF documents (which is no surprise). Apart from formatting, there really isn’t much there which can’t be found in Preview (built into OS X). It has the look of a Windows program ported to the Mac. Adobe would be better off releasing this program to the Linux or BSD platforms unless they plan on adding more features to it.
Stanza was quite a surprise. They claim it is the “standard” reader on the various “pad” devices. I can see why. There are two versions available. Naturally the commercial version has more features enabled than the free version. Stanza supports more file formats than the other two. It also supports “multi-column reading”. I wasn’t convinced about this until I tried reading an unformatted document. On a computer it is slightly better to go through this type of document than it is to read it page by page. On a pad I can see where this would be a great advantage. Bookmarks in particular are a good idea for readers. There is also limited support for themes in the free version. This area has some bugs but the work around is to select the theme then quit the program and restart it. Unlike the other two it can not deal with graphics. It is a text only program.
Skim has a slightly different focus. It can be used not only as a reader, but also as a way to take notes. I haven’t done much experimentation with this program but I can see immediately where it would be very useful for students and those who need to make notes in their manuals. Both times I made notes in a PDF file they appeared off to the side of the document. One left an icon on the main page and the other had a square I could double-click to open the note. In addition to the users notes, there is also the bookmark feature I mentioned earlier.
Each of these programs are currently available for free.