The correct title of this article is nxDoom. The initial letter is capitalized because of technical restrictions.
nxDoom is a source port of id Software's Doom engine to the Sega Dreamcast video game console by Stuart Dalton, better known as BlackAura. nxDoom is based on DoomDC by crt0 and DCGrendel. The first release was nxDoom Beta 1, which was released on August 11, 2002.
nxDoom is freeware, but the actual Doom game files are still commercial, and must be purchased. However, there are alternatives, such as free total conversions (mods which don't use any commercial Doom game files), and FreeDoom.
nxDoom supports the registered and shareware versions of Doom, The Ultimate Doom, Doom II, and Final Doom. It also has a modlist that allows you to select PWADs and DeHacked patches used to make mods and total conversions. It has VMU saving, and has sound and music when downloadable music WAD files are used.