"IDEA X" is the codename for IntelliJ IDEA version 10.
Download IntelliJ IDEA Ultimate EAP
IntelliJ IDEA 10.0.3
You need to have a valid IntelliJ IDEA 10 license in order to use this EAP build.
Download IntelliJ IDEA Community Edition EAP
Plugin Development Kit: Since the open-source Community Edition was released, we no longer provide the plugin development kit as a separate download. Please see http://www.jetbrains.org/display/IJOS/Writing+Plug-ins for instructions on setting up a plugin development environment.
EAP Build Announcements
The recommended way to get notifications about new EAP builds is to follow @intellijidea on Twitter. Alternatively, you can subscribe to the build announcements as an RSS feed, or create an account in Confluence and watch this page in order to get e-mail notifications about new builds.