The Compendium. I DM an online game using d20 Pro. When my PC's enter combat, I open each of their opponents in a separate Compendium window. When somebody uses a power, I can reference the Compendium to adjudicate it's effects.
I can also use the Compendium, along with google docs, to plan my adventures while I'm at work.
Dungeon and Dragon are intermittently fun to read. The quality of each is uneven, and I wouldn't subscribe just for them.
I use the monster builder only rarely, mostly just to level existing monsters up and down.
I don't give a damn about the character builder, and I'm tired of people bitching about it.