IntelliJ IDEA Community Edition is the open source version of IntelliJ IDEA, a premier IDE for Java, Groovy and other programming languages.
Fast, reliable - great tool with lots of features and support for myriads of frameworks.
Best Java IDE. Open source version lacks some frameworks support (Grails!) but still better then Eclipse/Netbeans.
Really good refactoring support. Super-fast find in files. Very framework aware (e.g. code completion from model in GSPs)