An excellent emulator.

Precise timing emulation synchronizes CPU with the cathode ray beam. Auto-detects pal/ntsc frame rates & cartridge memory banking types. Emulates joysticks, paddles, keypads, and steering controller." "Debugger with breakpoints, symbolic debug info (labels), automatic I/O and Kernal disassembler comments, built-in assembler/disassembler supports native/nocash syntaxes, upload to real atari function, help text includes full atari programming specs.