Alfresco: overview page on the Wiki; has pointers to useful looking guides, including Advanced Workflows - rolling your own workflow .
Alfresco uses jBoss' business process module(jBPM).
JBOSS
jBPM jump page : docs, downloads, fora, etc.
jBoss has a process design GUI available as an Eclipse plugin. GUI tutorial: Getting started: how to make your first process definition. , all the way through testing.