bee is an emulator of several video game systems. It currently emulates the "Sega 8-bit sytems" 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, it is available for Windows, Mac OS X, and Linux x86.
bee emulates hardware mostly at the signal level, which guarantees accurate timing but is computationaly costly. So don't be disappointed if the emulator does not run at original speed on your computer - you'll need a really fast CPU to reach full speed.
bee is free for personal use, but it's not public domain or open source.