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
Syntax highlighting for VIM is provided by Alessio Pace and is included with the current (7.1) VIM download, or is available separately here.
Here's a plugin that allows you to run scripts from within Vim. It was originally created for python, but it's easy to adapt to probably any language: http://www.vim.org/scripts/script.php%3Fscript_id=127
Download the latest version of the file, and save it in the /plugin folder of your Vim installation. Then edit it, and modify the line:
Be sure to have the groovy executable in the system PATH, then simply follow the instructions from the original site for the usage. If you wish, you can search for strings containing:
and change the value to set the size of the output buffer "pop-up" (F9).
TO-DO: Iimplement complete support for the groovy language, as has been done for ruby: http://wiki.rubygarden.org/Ruby/page/show/VimRubySupport
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.