Matt and I are co-DMing a campaign of Castle Zagyg. It is set in Greyhawk, using Yggsburgh as another independent city in the area.
The wiki for the campaign is at Obsidian Portal.
We started it by taking over DMing duties in a 1E AD&D campaign of which we were 2 of 8 players.
The whys and whynots of switching to Castles and Crusades are worth post that I may yet write. I was inspired by The West Marches Campaign and memories of my first experiences playing Dungeons and Dragons in a sprawling sandbox campaign also co-DMed.
Having players that were turned away from the initial campaign because of a lack of table space, we expanded the campaign to a second session. Each are played every other week. So far we have 13 players, one more to start. Some have multiple characters, and the DMs also play when they aren't DMing, for a total of 20 characters.