Think of MouseFest as a combination family reunion for people on boards like this one, and convention of WDW fans like us. It's held the second week of December every year and is split into two portions - sea and land. The sea portion is a 4-day cruise on the Disney Wonder, and the land portion is held at WDW.
MouseFest is organized by Deb Wills of AllEarsNet and Jen and Dave Marx, authors of the Passporter guidebook series, and it's sponsored by a large group of Disney-related webmasters, authors, and even a few business owners like travel agents and towncar companies.
Most of the events (commonly called meets) are free and don't require advance registration, though most of them are held inside the parks, so you need a park pass to get in. There are Hidden Mickey Hunts, trivia contests, photo tours, social gatherings to watch Wishes and Illuminations, meets to experience attractions as a group, group meals, and even some paid events like Segway Tours, and Illuminations and Wishes Cruises.
For more info, including a complete list of events and sponsors, check the MouseFest home page:
http://www.mousefest.org
I've been to MouseFest the last three years running (only the land portion, can't afford the cruise) and enjoyed every minute. You can read more about it in my trip reports:
http://willcad.org/disney_tr2003.html
http://willcad.org/disney_tr2004.html
http://willcad.org/disney_tr2005b.html