I'm aware of the built in code generation and refactoring one can do with the "Source" and "Refactor" menu items in Eclipse.
I also use the Commonclipse plugin to easily make use of the Apache Commons classes that build hashCode, toString, equals methods.
What other plugins or tools should I be aware of for generating Java source easily under the Eclipse IDE?
It really depends on what you want to do? J2EE, web services, Swing ...
I do a lot of web service development and rely heavily on:
For Axis2 development:
There's a good tutorial on Axis2 plugins called Developing Web Services Using Apache Axis2 Eclipse Plugins - Part 1
If your are interested in going a bit further with code generation and getting into model driven software development you should have a look at openArchitectureWare.
If you work through the Getting Started tutorial you should get up to speed pretty quickly.
I don't know if you're a big UML fan but IBM Rational suite is extremly powerful.
I used it to move from UML to generated mock objects but that's only a slight part of what it can do
I don't think that this is really what you're asking about, but for model driven development, I think Borland's Together is still the best eclipse based tool out there.
You can look at http://fast-code.sourceforge.net/ for spring based applications. You can create FooService and FooServiceImpl and the configurations just by typing foo. It has nice way to create unit tests as well. One can see all the templates here : http://fast-code.sourceforge.net/templates-config.xml