Gutenprint (formerly Gimp-print)

Tux Paint   (Compare Project)

License: gpl

grafx2   (Compare Project)

License: gpl

MyPaint   (Compare Project)

Mostly written in Python

License: gpl, lgpl21_or_later

VIPS   (Compare Project)

Mostly written in C

License: lgpl21

NIP2   (Compare Project)

Mostly written in Modula-2

License: gpl

Phatch   (Compare Project)

License: gpl3_or_later

PearPC - PowerPC Emulator   (Compare Project)

Mostly written in C++

License: gpl

printerd   (Compare Project)

Mostly written in C

License: gpl

Common Unix Printing System (CUPS)   (Compare Project)

Mostly written in Postscript

License: gpl

Fink   (Compare Project)

License: gpl