It may be necessary to know which license key is used by IDEA installations.

License key is stored in a file named idea.license (IDEA 3.x) or idea40.key (IDEA 4.x), or idea50.key (IDEA 5.x) (ideaXX.key where XX is IDEA version for the reset IDEA releases).

• For IDEA 3.x this file is located in IDEA system directory, which is by default ~/.IntelliJIdea/system, but may be different if the installation has been customized (~ stands for users's home or profile directory). To know the actual location, see inside IDEA_HOME\bin\idea.lax file, search for -Didea.system.path property.
• For IDEA 4.x the file idea40.key is located under ~/.IntelliJIdea/config directory. On your installation it may be different, see instructions for IDEA 3.x, the relevant property is -Didea.config.path
• For IDEA 5.x idea50.key file is under ~/.IntelliJIdea50/config by default. Actual location is set via idea.properties file  located in IDEA_HOME\bin directory.
• IDEA 6.x and later versions use the key from the same location as IDEA 5.x (~/.IntelliJIdeaXX/config), the file is named ideaXX.key, where XX is the version number (60 for 6.0, 70 for 7.0 and so on).

On Mac OS X license key file is stored under configuration directory: ~/Library/Preferences/IntelliJIdeaXX.

Please note that this file has UCS2 (2-byte Unicode) encoding and you can't view it with Notepad, you need to open it in some Unicode-aware editor. First line contains the key, second line contains user name.