I've blogged about it before, but I can't express how impressed I am by the work JetBrains have done on their Groovy plugin. Code completion, CTRL+click navigation from Groovy to Java and Java to Groovy, some refactoring support, joint compilation of Groovy and Java. JetGroovy really makes mixing Groovy & Java in the same codebase completely seamless.
How do you get setup? Follow these steps:
1) Download and install the IDEA 7.0 EAP from here
2) Download a stable binary of JetGroovy from here
3) Unzip the zip file into the IDEA_HOME/plugins directory
4) Start IDEA
5) Open up the JetGroovy preferences dialog in Settings/Groovy&Grails and point IDEA to your Groovy & Grails installs
6) You're done
Make sure you checkout the feature tour here. If you're not a IntelliJ user, don't forget their is also a lot of excellent work happening around the Eclipse plugin. Enjoy!