The code of the Intellij IDEA community edition is now available for semantic search and navigation in Codota!
With Codota, you can browse the source code of the project without having to load it into a local IDE.
For plugin developers this is particularly useful, as you can now learn how the IDE uses the same APIs you use as a plugin developer.
Check it out at http://www.codota.com/xref/#/intellij/findUsages
For example, here's list of all Usages for the AnAction class - http://www.codota.com/xref/#/intellij/findUsages?query=com.intellij.openapi.actionSystem.AnAction