NullDC is an open source freeware Dreamcast Emulator, developed by drkIIRaziel and ZeZu. While development of NullDC has now ceased, as the developers moved onto newer projects, and also started up a newer multi-platform Dreamcast Emulator Reicast.
NullDC is still one of the best Dreamcast emulators available, partially in thanks to its development style where the focus was to ensure the highest possible compatibility over performance. In the end this paid off well, with NullDC spouting some of the best compatibility among Dreamcast emulators.
NullDC also make use of a plugin system, so its functionality can be increased without having to actually rewrite parts of the core code, this was more useful back when NullDC was closed source.
Like an awful lot of emulators, NullDC does require the original Dreamcast Bios to actually be able to run anything at all, this is due to certain games relying on utilizing certain features found in the BIOS that can be difficult to replicate in code, especially if the games make there own temporary modifications to the BIOS.