IntelliJ IDEA Plugins
Nobody can ever hope to build a development tool that is all things to all developers. IntelliJ IDEA is no exception (though many people tell us that it is most things to many Java developers!) Right now our Plugin Repository lists more than 500 plugins, both freeware and commercial.
Install Plugins Using IntelliJ IDEA Plugin Manager
IntelliJ IDEA provides integration with Plugin Repository. A convenient UI helps you install any available plugin from the Plugin Repository without leaving IntelliJ IDEA. (IDE Settings | Plugins).
The IntelliJ IDEA Plugin Manager also supports automatically updating installed plugins when new plugin versions are released.
If you think IntelliJ IDEA is missing a plugin with a framework support or a feature, please submit a request to Plugin Wishlist.
Information for Plugin Developers
Setting Up the Plugin Development Environment
The recommended way of setting up a plugin development environment for IntelliJ IDEA is to check out the source code of IntelliJ IDEA Community Edition. The source code also includes a number of small sample plugins that you can use as a starting point for your development.
You can find the instructions for this at the setting up development environment.
A number of articles describing different aspects of plugin development and the architecture of IntelliJ IDEA can be found in the IntelliJ Platform SDK Documentation page.