Well I bought some degreaser from MEC (Mountain Equipment Coop). It's MEC's brand called "MEC Bio-Cycle." I'm sure it's probably the same thing as Simple Green or equivalent.
Anyways, it works great. I had a ton of grease and grime build up on my chain, cassette, and rear derailleur. Never had a problem taking off any of the grease. Worked like a charm. And yes, it is bio-degradable so the environmentally friendly stuff does work.
It's also cheap. $13 CAD for a 4 litre jug.