Good news for all Atari 8-bit musicians! The popular Theta Music Composer is now available as a cross platform tool under the name "enotracker".
TMC/enotracker supports all the features you can ask for, including full support for stereo POKEY setups. enotracker is SDL based, so it runs on Windows as well as on *nix systems. Download the alpha for Windows here, or check out the source on Github.