There are many plugins that are not part of the official gimp package.
They require to be compiled from C source.
For example: http://kcd.sourceforge.net/fix-ca.php
However, it is not possible to compile pugins for gimp.app.
See http://liquidrescale.wikidot.com/en:installation-instructions for explanation:
"the gimp.app project DMG distribution doesn't provide all the required headers...
2008-09-07 18:39:21 UTC in GIMP.app
