noegnud is a highly flexible user interface for NetHack and SLASH'EM. It supports ASCII mode, multiple 2D tilesets and hardware accelerated partially-3D graphics. As of now, the current version (0.8.4) allows playing not only under Linux, but under Windows as well. Since NetHack is a fundamentally two dimensional game, the 3D graphics are actually just the 2D tiles rendered as textures in a 3D world made by extruding the walls upwards.
As of 12 October 2014, noegnud is not available at its original web site; Clive Crous appears to have abandoned it in favor of Vulture.