abctool is a python script that manipulates music files in abc format (see the abc home page for more information on abc).
It's mostly useful for people working on the commandline and/or editing abc directly in an editor. Since it's written in python, you need a python interpreter installed. Although python is available for almost all systems, this might put some people off.
It relies on external programs for certain tasks like converting to PostScript and transposing.
It is open source, developed by Atte André Jensen and released under GPL.