Project AY - Music and software for the AY soundchip (as used in the ZX Spectrum 128K). - http://www.worldofspectrum.org/projectay/
STsndplay - A program for playing all those old Atari tunes without the need for an actual Atari to play them. - http://www.nocrew.org/software-stsndplay.html
DeliPlayer - A free audio player for Windows capable of playing AY files (and other formats such as SID, MOD and MP3). - http://deliplayer.com/
Lieves!Tuore - Home of MYMPlay. Available for MSX, ZX Spectrum and Amstrad CPC. - http://www.kameli.net/lt/
YM player and plugin for Linux - Download and installation information for the YM player and plugin for Linux. - http://www.cpu.lu/ulm/ymplugin.html
YM Plug-in for CL-Amp - YM Plug-in for CL-Amp for BeOS. - http://www.bebits.com/app/1628
SND Player - A specialized musicplayer for the 16/32bit Atari computers. SND Player can, as the name suggests play ".snd" files. These are songs using the YM2149 synthesizer which is fitted to all 16/32 bit Ataris and most Atari clones. - http://sndplayer.atari.org/