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.
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.