iSilo Document Reader (Popularity: )
http://www.isilo.com/ A freeware application for the PalmOS, PocketPC and Windows platforms. Supports the proprietary iSilo format.
Mentoract Reader (Popularity: )
http://www.globalmentor.com/ A freeware cross-platform application that works with standards-compliant works, such as .oeb and .html formatted texts.
TomeRaider (Popularity: )
http://www.tomeraider.com/ A freeware application for the TomeRaider format. Available for PalmOS, EPOC, PocketPC and Windows platforms.
uBook Reader (Popularity: )
http://www.gowerpoint.com/ A freeware, skinable application for the Windows and PocketPC platforms. Can handle .html, .txt, .rtf, .pdb and .prc formatted files.