From the documentation:  
  Stella is a freely distributed multi-platform Atari 2600 VCS emulator;
  originally developed for Linux by Bradford W. Mott. Stella allows you
  to enjoy all of your favorite 2600 games once again by emulating the
  2600's hardware with software. Stella is written in C++, which allows
  it to be ported to other operating systems and architectures. Since
  its original release Stella has been ported to AcornOS, AmigaOS, DOS,
  FreeBSD, IRIX, Linux, MacOS, OpenStep, OS/2, Unix, and Windows.

WWW: http://stella.sourceforge.net/