Three recommended sturdy and free serif fonts

[IceHand]

I was looking for sturdy and free serif fonts that look good at the relatively low resolution of the Cybook and found some I would recommend:

Fontin: A modern looking semi-serif font that is especially suited for small sizes. [link]

Vollkorn: "Vollkorn" is German and means "wholemeal". True to its name the font has only small variations between thick and thin strokes and thus is very usable at small sizes and for small resolutions. It has a very classical Antiqua look, but with modern variations. [16.11.2010: updated to v2.1]

IM Fell Great Primer: A very interesting font as it uses cuts from 1684 while retaining the original look as much as possible. The italic version is a bit hard to read and the brightness of the characters varies a bit, if one looks closely. Nonetheless a usable font especially to give e-books a historical look.
Digitally reproduced by Igino Marini and released as a free font with some restrictions. No bold version. I only changed the line spacing.

[IceHand]

Two more recommended free serif fonts:

Grandesign Neue Serif (download link at the upper right)

Droid font family from Google & Ascender (download link near the bottom of the page)