Float Plan - Crew a yacht in exchange for passage. This is a classifieds add aimed at finding a yacht to work on, or for yacht owners to find crew when they're short a person or two.
http://www.floatplan.com
Find a crew - I like how this website has categories such as 'compentent crew' or 'romantic partner' so you know little better whatt you're getting into before contacting the person.
www.findacrew.net
Boating OZ - Good ratio of boats availiable to crew wanted.
Non-OZ based trips as well.
http://www.boatingoz.com.au/messageboard/?mb=j/lFhAjA