Separate+ is a CMYK Separation plugin for The GIMP.

One thing preventing The GIMP from being useful
in a pre-press environment is the lack of support
for the CMYK colour-space. This plug-in goes some
small way towards rectifying the situation, using
a trick with layers to fake CMYK support.

WWW: http://cue.yellowmagic.info/softwares/separate.html