So you want to build a world or a dungeon? Three (okay, four) posters are doing exactly that right now in very different ways.
Douglas Cole is getting a crowd-sourced world build going for Dungeon Fantasy. Want to design part of it? Take a look.
GURPS Hexworld Challenge
Slow Dungeon Building
Matt Riggsby is building a DF dungeon bit by bit:
The Occasional Dungeon
It's much like the approach of Castle Triskelion.
Build As You Play
Justin Aquino is posting about something I prefer to do - build as you play.
World Building as we Play
In the post is a structure of what to decide firmly ahead of time, and what the players bring to the table through character generation, and what gets built afterward. Justin is really big on systems - systematic approaches to solving gaming-related issues of design, play, and organization. I tend to be more loose about it all, but having a system you can replicate and share is a good thing.