Tara Labs using a vacuum dielectric. Nordost and Harmonic Technology use air. I'm waiting for someone to use sodium hexaflouride. This gas absolutely will not let electrons pass through it, is non-toxic and inexpensive. sounds worthy of interest.
the air dielectric
I don't know what the first company was that used a nearly 100% air dielectric in their cables. Slinkylinks appears to have been doing this all of 20 years ago. These days, Inakustik stands out to me for the explicitness and full depiction of their careful internal construction. Something that Skogrand doesn't do. At the more inexpensive end of the scale, Black Cat Coppertone Flatwave has found a way of implementing this nearly as well but with a lot lower manufacturing costs.
Other brands claim an air dielectric (e.g. DH Labs) but I have yet to see photographic proof.
Who else is really using an air dielectric these days?