Lxdream is a free and open source emulator for the Sega Dreamcast system, running on Mac OS X and Linux.
While Lxdream is still in heavy development (and many features are unimplemented or buggy), it is capable of running most demos and some games.
Edit, script, and test places with advanced tools.
Opens applications with small texts sizes.
GameRanger is the easiest way to play your friends online.