Open source OCR Engine developed by HP Labs - now improved by Google
The Tesseract OCR engine was one of the top 3 engines in the 1995 UNLV Accuracy test. Since then it has had little work done on it, but it is probably one of the most accurate open source OCR engines available. The source code will read a binary, grey or color image and output text. A tiff reader is built in that will read uncompressed TIFF images, or libtiff can be added to read compressed images.