AllReader Plus: really the best eBook reader by far! So many option and features. Only downside is it doesn’t support PDF files so you’ll need also another reader if you don’t want to convert your pdf files first to txt.
RepliGo: it’s good for reading pdf files with a lot of images (otherwise difficult to convert properly to txt) but personally I’d use the already installed and free Adobe Reader 2.5 LE (or 2.0)
uBook: can read many formats and seems to be good with documents that have images in them, like PDF; very bad interface. (NOT TESTED MYSELF YET)
Foxit Reader: another reader for PDF, I read some positive comments (NOT TESTED MYSELF YET)
PocketXpdf (freeware): a viewer for native PDF files (NOT TESTED MYSELF YET)