I wonder the extent to which string quartets or a cappella choirs are non-ET. They are often cited as examples of the tendency towards just intonation when not using fixed-frequency instruments but I'd be interested to know if that's demonstrably true in certain recordings, etc.
I'm temped to say that all professional strings, winds, and choirs do this to some extent--you'll just be able to hear it with more clarity in small ensembles like a string quartet. A string quartet that played out of tune simply wouldn't be considered at a professional level. My trombone quartet, for example, worked on this to great extent just as an undergraduate ensemble at a music school.
If you're looking to make comparisons, you'd have to judge a professional string quartet recording with a MIDI version. And of course, the music is still all written with temperament in mind--everything is going to be grounded in contemporary methods of tuning; it's just that professionals can use their ears to eliminate beats from any given chord.
There is only one composer I'm familiar with who writes in Just Intonation. Toby Twining has released three albums, each 10 years apart, all using extended vocal techniques and alternate tuning systems. I HIGHLY recommend the Chrysalid Requiem to anyone interested in Just Intonation. (Hear previews on iTunes)
There are probably other composers, but this work in particular I find especially approachable by ears that have been steeped in traditional tonality. It is an a cappella choral masterwork incorporating Tuvan throat singing and justly tuned microtones that are so challenging to perform that the already incredibly talented singers require a synthesizer track in their ear to tune to as they sing. I see it as the most important musical work of our era (but I'm admittedly a little strange). Check it out.