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.
Vim provides Groovy syntax highlighting out of the box (current release), as provided 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.
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.