formerly known as Groovy on Rails, a name giving a good hint what Grails is all about.
activiti plugin integrates Activiti BPM, Google Code
Alfresco includes Activiti BPM
camunda plugin integrates camunda BPM, GitHub
Apache Camel aka routing plugin offers messaging incl. JMS and allows integrating numerous external systems
Arrested generates RESTful controllers for domain classes and maps them in UrlMappings, generates single-page AngularJS-based views, and provides simple token-based security
Vaadin
ZKGrails to build ZK-based UIs
Automated Business Logic (ABL) complements the Grails attribute validation framework with support for efficient multi-table derivation / constraint logic. This integrates via GORM events, so all existing Grails APIs and services for testing, services, etc continue to work without changes.
Executor will run asynchronous / concurrent threads
GVM the Groovy enVironment Manager
hawtio is a modular web console for managing your Java stuff
Jesque is an implementation of Resque in Java, Jesque Web
Microsoft Excel files: Apache POI
Microsoft Word files: docx4j
Quartz plugin adds job scheduling features, Quartz Monitor
JSON Schema; JSON Editor; JSON Editor Online (GitHub)
Metawidget, related posts at Kennard Consulting's Blog
CalDAV, CardDAV, WebDAV