All of the newer Sony players of the last couple of years, Archos 5 & 7, Creative Zen X-Fi and Creative Zen (but not the Zen MX) that I know of.
Is there a list of players other than the iPod which support the m4a format? I have recorded a large song collection with iGetMusic (http://www.amphony.com/igetmusic/index.htm) which is in the Apple m4a format. Converting to MP3 would double the file size of the songs.

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