Boards

Have got me through most of the essays I've written in the last couple of years.

Tortoise
Pele
Music For Airports
Stars of the Lid
Earth

but they've been said haha. lateral noise by god is an astronaut is my track of choice for work. unobtrusive but still there

for me it doesn't really matter if it's heavy and rocking or utterly mellowed out if it's something I've never really listened to I'm more likely to listen to the music than do the work.

So even if it's something intense like Clutch as long as I'm familiar enough with it I can tune it out if I need to and can happily tap away to the rhythm without the music ever becoming too intrusive.

also,
gas - pop/konigsforst
grouper - cover the windows and the walls
fennesz - black sea

The new one (Vivian & Ondine)is very good.

Is my stock reply for this question, I stick by it.

got a lot of rotation when i'm reading.

Boards of Canada, and as mentioned, Steve Reich

By the time you get to the end you forget what the beginning sounds like - get through it at least 3 times without getting bored.
Failing that, the Basic Channel stuff, although I find that a bit more immersive (which is why I personally wouldn't want to listen to Stars of the Lid, and tend to switch everything off, stop working, and at most, drink a glass of wine or something when I listen to ambient music)

So, I reckon minimal techno is a good way to go whereas for me, ambient isn't because I stop concentrating on the work and go into little daydreams, and, if it's good ambient, it doesn't really do the music itself enough justice either.

Classical stuff as well, Steve Reich, Philip Glass; something minimalistic with a bit of rhythm.

Lately I've been going for Max Richter - 24 Postcards In Full Colour when writing/ designing.

So the credits for finding this are not for me, but for someone else, it's beautiful! http://www.youtube.com/watch?v=C_ILgJEWoIk

Health's debut while studying at uni, the second half the album scares the shit out of you when you're concentrating on something else, which in turn has the added effect of making you focus better, true fact.

Easily the album I work best to, although This Will Destroy You has also been on whilst I've been in productive mood...

and a strong coffee

CODY or Happy Music...

it's so driving and repetitive that it's as if it does some of the concentrating for you. after you've listened to it a couple of times/listened to a long version of it you will HAVE to listen to something else tho, at least my brain could only take about 40 minutes of having a C major chord pounded into my ears.

Not necessarily The Field, but his music under Porte or Lars Blek.
GAS
Radicalfashion

Music from the free 'Silent Ballet' compilations from Lost Children net label.

This guy is soo good. Kinda 80's pastiche music. It's all slightly tongue-in-cheek but just very innocent and naive. A bit like M83's lonely quiet brother who sits in the library. And, like you are looking for, it is VERY unobtrusive, perfect background music.

I've been busting out loads of code to the Moderat album, also Ellen Allien + Apparat's Orchestra of Bubbles.
Fuck Buttons/Health are also pretty good,

Nick Drake is always a winner. Elliot Smith too.