If you're a teacher using Greenfoot, or thinking about it, take the time to sign up for access. Access is limited to those who can show some bona fides to show that they're teachers, not sneaky students trying to wreck the teacher's lesson plan. ;)
That's what kept me from signing up for some time, I just didn't take the time to dig up something with my name on it at my school's website to show my status. Well, I finally took the time to do it, and now I wish I'd bothered a long time ago!
The resources available include a wide selection of very well presented lessons, exercises, and information on using Greenfoot with Eclipse, etc., etc. Then there's the discussion boards. I follow the Greenfoot and BlueJ mailing lists, but the Greenroom discussions have an extra dimension to them, particularly from the teacher's perspective.
If that's not enough, that's also where to find the announcement of the forthcoming Greenfoot 2.0! My excitement about Greenfoot 2.0 is the new editor, with syntax highlighting and code completion. I can't wait to get my hands on it, I'll post about it when I do.