The World of Trees. I made the core world by hand using MCPaint. We have hosted this on our private creative server for quite some time now. Lots of people have added on.
My brother and our friends are slowly building a medieval city sprawling outward from the original spawn point.
We did play this as survival for a while back when it was first hosted but the world was too devoid of iron to really have a good time. Now we have a railway going into the naturally rendered survival areas and all is well.