There is a patch (known just as the Lethe patch) available for Nethack that adds the river Lethe and other netherworldly interesting things to the game. The patch was written by Michael Clarke.
The river (complete with ?SoA effects! =) starts from under Medusa's level, and leads to the gates of Gehennom. This addition isn't the only one - Gehennom has got some pretty neat additions and changes, and reportedly it's much more interesting part after that. The special demon levels are bigger. The Castle, formerly the gateway to Gehennom, is now just another level in the "Lethe Gorge".
The patch adds new scrolls, too, bunch of new monsters (drawn from Cthulhu Mythos), and blunt silver weapons (for those who have religious problems using bladed weapons but would pretty much like to deal with demons properly, too).
I might try it, if I only ever would get that far in the game...