Still in time for the plugin contest, but much later than anticipated, I present Lineage 2.0
Position the cursor on the class or reference to a class you want to subclass and press Alt+Enter. An "Extend Class" intention appears, which when invoked, allows you to specify name, visibility (default: public) and destination package (default: the package from which you invoked it). All fields and button are accessible via the keyboard for the highest efficiency.
Easy subclassing by inner class:
Creating an inner subclass of another class is just as easy as creating a regular subclass. Press Alt+Enter on a class or reference to a class and invoke "Extend Class with Inner Class". The Extend Class dialog now allows you to specify a destination class (using IDEA's familiar Class Chooser dialog) and if the created class should be declared "static".
Highlight Usages in File for "import":
Invoking highlight (default CtrlShiftF7) usages on the "import" keyword, will highlight all references to the class imported in the open file. When the import statement is an "on demand" import, a chooser will first popup, allowing you to choose which class to highlight or to highlight all imported classes.
Highlight Usages in File for "extends" and "implements":
Easily highlight all implementing or overriding methods in the file by invoking Highlight Usages in File on an "extends" or "implements" keyword. If necessary a chooser will pop up where you can specify from which class you want the implementing/overriding members highlighted
This plugin is compatible with IntelliJ IDEA 6.0 and can be found in the Available list of the Plugins dialog in the Settings of IntelliJ IDEA. Alternatively it can be downloaded from: http://plugins.intellij.net/plugin/?id=104