GNU Ocrad is an OCR (Optical Character Recognition) program implemented
as a filter and based on a feature extraction method. It reads a bitmap
image in PBM format and outputs text in the ISO-8859-1 (Latin-1)
charset. It can be used as a stand-alone console application or as a
back-end to other programs.

gocr is another interesting command line OCR tool. Both can be plugged
into Kooka, the KDE scan and OCR program.