XMAME (a.k.a XMame or x-MAME) is the port to the *nix world of the most excellent MAME program. It is open source (non-GPL) software maintained by a small bunch of enthusiasts. It is derived from the MAME source code, with additional code to support a very wide range of *nix platforms.
More information can be found at the official Web site (x.mame.net).