nullDC is an open source Sega Dreamcast and NAOMI emulator for Windows, developed by drk||Raziel (currently under the nickname skmp) and ZeZu. It was released under the MIT license.
nullDC requires DirectX 9.0c, Visual C++ runtime libraries and optionally WinPcap for modem emulation. BIOS files are also needed. It has a plugin architecture, with several alternative implementations (some ported from Chankast) for graphics, sound, reading games burned to CD-ROMs (it cannot read GD-ROMs directly) or disk image files, memory cards, etc.