IntelliJ IDEA 9/10, what folders to check into (or not check into) source control?

We have a FAQ article covering this question.

[The .idea] format is used by all the recent IDE versions by default. Here is what you need to share:

• All the files under .idea directory in the project root except the workspace.xml and tasks.xml files which store user specific settings
• All the .iml module files that can be located in different module directories (applies to IntelliJ IDEA)

Be careful about sharing the following:

• Android artifacts that produce a signed build (will contain keystore passwords)
• In IDEA 13 and earlier dataSources.ids, datasources.xml can contain database passwords. IDEA 14 solves this problem.

You may consider not to share the following:

• .iml files for the Gradle or Maven based projects, since these files will be generated on import
• gradle.xml file, see this discussion
• user dictionaries folder (to avoid conflicts if other developer has the same name)
• XML files under .idea/libraries in case they are generated from Gradle or Maven project

.idea directory is a replacement for the old .ipr (Idea Project) file and if you want to share the project between users, then you need to share .idea folder (with the exceptions mentioned in the FAQ) and all the .iml files.

Refer to GitHub's JetBrains.gitignore file to always have an updated listing of which files to ignore.

Not an exact answer to the question, but there are sample .gitignore files available here, including one for JetBrains which includes IntelliJ.