Hatari is an open source Atari ST/STE/TT/Falcon emulator for Mac OS X, Windows and Linux environments.
It offers hardware emulation with some other features such as joystick emulation and mounting of host’s directory as an ST drive, which is handy for copying files to Atari virtual machine that you set up.
Hatari comes with EmuTOS, which as a free replacement for Atari’s TOS. If you have original TOS images from your Atari, you can use those instead.
Hatari has also been ported to other systems, such as Nintendo Wii or AmigaOS – check out this page.