A NetHack variant designed to feature more randomness, more levels, more challenges and more fun than vanilla NetHack. Changes include better graphics, improved user interface and several of the popular patches. It is a bit easier on beginners than NetHack although it still might slaughter you unfairly from time to time (to keep the original NetHack feeling).
If you're looking for something as close as possible to vanilla but without the soul-searing pain of unfunness that is vanilla's Gehennom, I'd go UnNetHack. I can't express it well, but UnNetHack captures the same feel of the best parts of vanilla very well.