OCS.

gocr.

$ gocr $IN.pnm >$OUT.txt

ocrfeeder.

Document layout analysis and optical character recognition system:

$ sudo apt-get install ocrfeeder

Using:

$ ocrfeeder-cli --o $OUTDIR --format HTML --images $IN.pnm

tesseract.

Installing:

$ sudo apt-get install tesseract-ocr

Using:

$ tesseract $IN.tif $OUT
$ cat $OUT.txt

ocropus.

$ ocropus hocr-to-text screen.ppm

ocrad

Optical Character Recognition program:

$ sudo apt-get install ocrad

Misc.

unpapper