There is a Groovy plug-in for the popular TextMate editor on Mac OS X here.
Brian McCallister has created a plugin for SubEthaEdit, which is here.
A new-format language "mode" for SubEthaEdit 2.x is available here.
A Vim plugin is available here (written by Jim Ruley).
Vim provides Groovy syntax highlighting out of the box (syntax file created by Alessio Pace). The syntax file is also available here.
Guillaume Laforge wrote a syntax file for the TextPad text editor which can be downloaded
here. This file should be installed in the Samples subdirectory of your TextPad installation.
Michal Szklanowski contributed a Groovy 1.0 syntax file for the EditPlus text editor which can be downloaded here. Groovy 1.5 syntax and auto-completion library can be downloaded from here.
See the Emacs Groovy Mode page for further details.
Find more info on the UltraEdit Plugin page.
Syntax file for Ps PAd is available here by Marc DeXeT.
State file for GNU enscript. It needs to be installed alongside the other enscript state files, for example in
/usr/share/enscript/hl/ on SUSE linux.
Language definition file for Highlight.
Syntax files for Crimson Editor are available here (provided by Jim Ruley).
Syntax file for Kate (KDE4) based on the Java syntax file is available here.
To install the new syntax file for the system, place it in /usr/share/apps/katepart/syntax.
To install for your user, place the file in ~/.kde4/share/apps/katepart/syntax.
Paths are valid for Arch Linux and may have to be tweaked for other distros.
A simple script that helps you get to your Grails artifacts fast from the base of your project with the command line using Gedit.
The script is called gred, and is available here on Github.