This document is out of date, please refer to the new version: http://intellij-support.jetbrains.com/entries/23393067.
Depending on the IntelliJ IDEA project format (.ipr file based or .idea directory based), you should put the following IntelliJ IDEA project files under the version control:
.ipr file based format
Share the project .ipr file and and all the .iml module files, don't share the .iws file as it stores user specific settings.
.idea directory based format
Share all the files under .idea directory in the project root except the workspace.xml and tasks.xml files which store user specific settings, also share all the .iml module files.