Categories are tricky. For some people, "free jazz" means music that is composed on the spot. For others, it's a synonym for avant-garde jazz, incorporating compositions that feature strange harmonies and tricky rhythms to form music often sounds improvised. eMusic doesn't have a separate category for free jazz, so neither will I. But you can find plenty of music that fits the first definition, as well as that of the second, under the "Avant-Garde Jazz" category.
These 12 albums stretch from the beginnings of avant-jazz with Albert Ayler, Sun Ra, and Cecil Taylor to the modern-day experiments of Ken Vandermark, Henry Threadgill, and David S. Ware. Together they're part of the outsider-jazz continuum: new music made by special people for specialists.
But given the chance, there's no reason that recordings like these can't appeal to those outside the jazz-nerd ghetto. Radiohead fans will find much to love in the sounds of Cuong Vu. Funk fiends should dig the grooves of William Parker. Fans of ambient music and modern classical can bliss out to the blips of Trapist.
Because what free jazz did, other than upset the jazz mainstream, is help change the way music of all kinds is heard and played. For instance, the extended musical techniques of '60s jazz became a standard part of acid rock, and free-form jams became de rigueur for exploratory bands wielding guitars, not saxes. (Think Jimi Hendrix, Pink Floyd, MC5, etc.) Since then, a whole generation of creative musicians, from rock to electronica, have absorbed the liberties that free-jazz introduced more than 40 years ago.