I love working with instrumental metal, typically prog or "djenty" stuff, in the background. Sometimes a bit of post-rock.

I was surprised how many of us there are. A lot of "professionals" I know listen to metal. Engineers, developers, IT folks. It will randomly come up and everyone will say something along the lines of "Oh, really, you too?"

Been listening to a lot of Their Dogs Were Astronauts recently. In the interest of sharing my personal favorites bands to work to, because I found a number of them from random mentions in forums:

• Their Dogs Were Astronauts -mostly instrumental.
• Wide Eyes - mostly instrumental.
• Caligula's Horse - instrumental only tracks available.
• Polyphia - mostly instrumental.
• Protest The Hero - instrumental only tracks available.
• TesseracT - instrumental only tracks available.
• Apocalyptica - started covering Metallica as cello quartet instrumentals, now has a drummer and a number of original songs, often with guest vocalists.
• Collapse Under The Empire - post rock, mostly instrumental.
• Parhelia - post rock, mostly instrumental.
• Opeth - occasional instrumental tracks, started as death metal, evolved into more prog metal. Huge fan of most of their work in general.

Edit: I'm stuck at work with my PC being worked on so here are some more random artists whose music I appreciate:

• Anup Sastry - very djenty, mostly instrumental, some guest vocals occasionally.
• Scoredatura - djenty, mostly instrumental.
• Rishloo - psychedelic prog rock? not instrumental.
• HAKEN - instrumental only tracks available.
• Brass Against - brass and other classical instruments. They cover Rage Against The Machine as well as other random songs and make occasional mashups of them. Not instrumental.
• Pomegranate Tiger - prog metal.
• Modern Day Babylon - djenty, instrumental.