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Struts 2 (if you want to use the OGNL support).
(These plugins are bundled with the IDE and enabled by default.)
See Managing Plugins.
Don't forget to add your libraries to the dependencies of corresponding modules.
Overview of Tiles 3 support
Tiles 3 support in IntelliJ IDEA includes code completion, and syntax and error highlighting for:
Tiles definition files
Expression languages (EL): Unified EL and Object-Graph Navigation Language (OGNL)
Tiles - JSP support tag library