Subversion is very popular among JDeveloper users, for several reasons, which are detailed as follows:
It has been supported in JDeveloper for a long time
It's widely used—lots of other people are using it, and many other tools can read your Subversion repository
It's free—always a good point
It's atomic—either your whole commit goes into the repository or nothing does. Since ADF projects consist of many interdependent files, this is very much desirable
To use Subversion, you need a Subversion server and client. The Subversion server is available for all platforms—if your version control server is based on Microsoft Windows, you can use the VisualSVN (http://www.visualsvn.com/server), which comes as a standard Windows MSI install file.
JDeveloper comes with a Subversion client for working with code, but if you keep other files in Subversion as well, you probably want a standalone client to update and commit to the repository...