bee - The Multi-Console Emulatorbee is an emulator of several video game systems. It currently emulates the "Sega 8-bit systems" Master System, ColecoVision, and SG-1000, as well as the Atari 2600/VCS. More systems will be added in the future.
bee is designed to be portable, accurate, and easy to use. At this time, the program is available for Windows, Mac OS X, and Linux x86.
bee emulates hardware mostly at the signal level, which guarantees accurate timing but is computationally costly. You'll need a reasonably fast CPU to reach full emulation speed for all systems.
bee is free for personal use, but it's not public domain or open source.