The Castle of the Mad Archmage picks up where Gary Gygax's Castle Zygag: The Upper Works by Troll Lord Games left off (since they lost the license from Gygax Games after Gary passed away) - at Level 2. Essentially, it's the dungeon below the ruins of Castle Greyhawk. Joe Bloch (the Greyhawk Grognard) has done an amazing job collecting Gary's (and others who played in the original) comments and hints about Castle Greyhawk, and is designing his own version based on those. The PDF, which gets updated with a new level regularly, is a great read - Joe Bloch has really captured the "feel" of an old school TSR module and it includes a lot of Gygaxian prose and even some humor. It's a free download, so I recommend checking it out.
And if you don't have Castle Zygag: The Upper Works, you should check out Rath's (Richard Graves) Level 1 of The Mad Demigod's Castle.