On this page you can find two fine extensions on SQL Developer.
With ErrorsLookup4Raptor a small entry field is added to the toolbar where you can enter an Oracle error code. The cause and action of the error will be displayed. Saves a lot of time searching through manuals!
The other extension is Insider4Raptor, a graphic tool that shows all your database activity and traffic. Looks a lot like SQL Monitor from Quest, but a lot cheaper (like free Laughing). Take a look at this screendump (in real life there are a lot of "moving dots" and it looks even more fancy!)
Try it out yourself!