Over the past year or so, I've heard of a website for running various pen-and-paper role-playing game campaigns called Obsidian Portal. I know Gabe of Penny Arcade wrote about it awhile back, and it caught my eye. Now that I'm running a campaign of my own, I've been looking into it more closely.
What a damned cool tool. Setting up a wiki for your campaign is a great idea that makes prep flow naturally (make pages for characters, locations, factions, etc. and flesh them out later), and your players have full access for creating adventure logs, character backgrounds, browsing personal forums, and all for the base price of free. Even if they don't use it, it's still extremely useful, and way impressive. And I love being able to put notes on every page that are only visible to you. It's a rather great way to game.