Download extension and copy to SQL_DEVELOPER_HOME/sqldeveloper/extensions/ and run SQL Developer.
Note for Mac OS Version: copy extension to SQLDeveloper.app/Contents/Resources/sqldeveloper/sqldeveloper
In the menu “File” you will see new menu item “Show Me Database Password”
If you click on menu item “Show Me Database Password”, then displays a window that contains one table with three columns: connection name, user name and decrypted password:
http://code.google.com/p/show-me-password-sql-developer/downloads/list