So what's so special about Masterplan and what it does anyway?
Since the lates 8.0 version it is called Adventure Design Studio and it's real good name for this.
The main screen of Masterplan looks like this
All the yellow rectangles are plot points which can be anything you want in you adventure - combat, skill challenges, NPC interactions, vingettes, quests and so on.
You can connect them any way you want and they automatically line up, making a neat flowchart you see above. Every plot point can be filled with detailed description and all the game stats you need. What's neat is that Masterplan helps you in mechanical stuff - which DCs you challenge needs for this party level, what's the difficulty of the encounter with those monstes will be and so on.
The program allows you to build tactical maps from tiles and then use them as maps for your plot points with different overlays and tokens.
Every plot point could be expanded to create nested plots with any level of complexity.
And another wondeful thing is that if you have a DDi subscription you can download monsters, traps and magic items from DDi Compendium and use them in Masterplan.
In short - Masterplan allow to keep all your adventure notes in one place, without needing anything else.
I highly recommend it to any 4E DM who thinks about going digital at his table.
And here are some more pictures.