Although a late 2nd Edition module, the Labyrinth of Madness was the greatest 20th level game I've ever played through. I'm away from home at the moment, but several years ago I converted a large majority of the encounters to 3.0
The module is tagged for levels 15+, but my party in 2nd Ed played at 20, and we had a few deaths.
If you can get your hands on the module itself, I'll e-mail my "conversion" to you later.
Just for a taste of the kind of things the players encounter:
A room that slowly changes everything on a person's body to iron, and then subjects them to Heat Metal. Very problematic if one of the items changed to iron is your pants - makes it difficult to leave the room.
One of the first encounters is against 8 invisible trolls that have long metal rods with black globes on the end. They smash the globes into the PC's as their fist attacks, thus breaking the globes and releasing the black puddings inside...
It's also a great adventure, because the Labyrinth changes and alters as the PC's advance through it, gaining sigils that allow them interact with new aspects of the labyrinth that simply did not exist for the PC's until they got the correct sigils.