TextMate (Mac OS X Only)
There is a Groovy plug-in for the popular TextMate editor on Mac OS X here.
XCode (Mac OS X only)
A new-format language "mode" for SubEthaEdit 2.x is 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.
Jeremy Rayner has written a groovy-mode for emacs, details of which can be found on the Emacs Plugin page.
Russel Winder has started an alternative to Jeremy's based on CC Mode (Jeremy's was reworking of ruby-mode). Also Stuart Clayman has created a Groovy execution mode. See the Emacs Plugin page for details.
Find more info on the UltraEdit Plugin page.
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.