My favorite is Palmone's eReader Pro.

I have used it for several year and I love it.

The eReader bookstore has a wide selection of currently popular titles, making it easy to find a variety of books to take with me on one of my many business trips.

The eReader software supports a variety of fonts and colore schemes, allowing me to customize the reader to make it easy on my eyes. I also like the abililty to use bookmarks to save my place, and to annotate by adding my own personal notes, when desired.