JetBrains, the makers of the award winning IntelliJ IDEA Java IDE are working on a Groovy and Grails plugins. The plugin is currently in the making and the official plugin homepage can be found on JetBrains wiki.

Download 

Attached are compiled alpha versions:

groovy-selena7020-v8866.zip,
groovy_selena7027_rev9073.zip,
groovy_selena7051_rev9366.zip,
groovy_selena7065_rev9435.zip,
groovy_selena7104_rev9911.zip,
groovy_selena7255_rev10441.zip.

JetBrains also seem to provide their own stable builds.

Download the appropriate Selena EAP version from JetBrains and unzip the attached file into your plugins dir.
Please be aware that this version is not yet supposed to be stable (but it works pretty well for me --Dierk).

Features

The JetGroovy plugin provides a rich set of features for both Groovy and Grails development. Here, we're listing the main features, but you can find out more on JetGroovy Groovy features page and the JetGroovy Grails features page.

Editing

• Syntax highlighting
• Error highlighting
  o Unresolved classes marked as errors
  o Unresolved unqualified properties marked as warnings
  o Incompatible types assignments marked as warnings
  o Inapplicable method calls marked as warnings
• Code folding
• Code formatter
• Comments and "To Do" view support
• "add missing import" intention

Execution, Debugging & Compilation

• Debugging support (setting breakpoints, view stack)

• GDK configuration
• Grails configuration
• Run configuration for Groovy scripts
• Make or rebuild java+groovy project entire

Code Completion

• context sensitive keyword completion
• Completion of methods, fields, classes
  o Completion makes use of type inference
  o Default methods completed
  o Swing builder methods are completed, swing properties are completed in method argument list
• Resolve
  o Resolve groovy to groovy classes, methods, fields
  o Resolve groovy to java classes, methods, fields
  o Resolve java to groovy classes, methods, fields

Surround with (Alt+Ctrl+T)

• if () {} / else {}
• while () {}
• try / catch / finally
• closure {}
• (Type) expr

Introduce variable (Alt+Ctrl+V)

• Introduce variable
• Offer the default type and variable name

Structure view

Shows

• classes
• methods
• fields
  found in groovy file

File structure (Ctrl+F12)

• The popup window contains classes, methods, fields.

Find usage (Alt+F7)

• Implements groovy classes search

Class search (Ctrl+N)

• Searches class by prefix of class groovy name

Grails

• Grails SDK configuration
• Grails controller, view, domain class, job, script, service, taglib creation
• Grails generators are built in; output to console

Wish list

A user wish list is available: