SQL Developer is a great tool for building database applications.
Oracle JDeveloper is a great tool for developing Java applications (with Oracle ADF or plain JEE), and it has some database development features that it inherits from SQL Developer.
However, one SQL Developer feature has been missing from JDeveloper until now: The ability to monitor database sessions. As any developer knows, this can be really useful both while debugging and while performance tuning. Because of this lack, I've been running both SQL Developer and JDeveloper concurrently, straining my development machine (which is also running a WebLogic and a database).
But now JDeveloper monitors database sessions! As of JDeveloper 12.1.3, on the Tools, Database menu, you find a Monitor Sessions menu item. This seems to work just like in SQL Developer 4.0 and includes the options to trace and kill sessions. When you choose this menu item, you are prompted for a database connection - make sure to choose one with DBA privileges, as it needs access to V$SESSION and maybe other system views. If you get ORA-00942 when selecting this menu item, it means that the database connection you chose cannot find the system objects it needs to present the session overview.