Developers should head staight to http://mule.mulesource.org/wiki/display/MULE/Home

For the developer wiki, list of mailing lists, irc channels etc.

Before working with Mule, understanding the concepts described at the following sites is important.

Enterprise Integration Patterns homepage - http://www.eaipatterns.com/
SEDA explanation - http://www.eecs.harvard.edu/~mdw/papers/seda-sosp01.pdf

The MULE Developers Guide is used for people wanting to work ON Mule - http://mule.mulesource.org/wiki/display/MULE/Developers+Guide
The MULE Programmers Guide is used for people wanting to work WITH Mule - http://mule.mulesource.org/wiki/display/MULE/Programmers+Guide