As some people may know I have a project on sourceforge for a couple examples on what could be created in an extension. The examples I had out there for Excel via Apache's POI project and support for XMLType support are not that useful anymore since the base sqldeveloper 1.1 now has both of those in it. So, I'm in the process of getting a couple other examples out there which will be roughly based on the presentation from OOW.
The first extension I've done is pretty simple but makes the tool look much nicer on OSX by adding Quaqua as a choice for the Look-and-feel. Non-OSX users can also install this extension and get the UI. Also only on OSX, it adds some support for Growl when a connection is opened or when a script is run. The sources for these are in the esdev project in subversion .
For those on OSX this screen with the blocky buttons probably looks quite normal.
To add these in just do a check-for-update and add http://esdev.sourceforge.net/center.xml as a new update center.
After loading the extension, go into the preferences and there's a Look and Feel Combo box. Choose Quaqua and restart.
With Ququa loaded the screen looks much nicer.
The same could be done to load some of the other LAFs out there like napkin mentioned for jdev by Shay.