sigur ros
dilute
explosions in the sky
radiohead
a silver mount zion
autechre
can
king crimson (some)
coldplay
kraftwerk
orbital
theres plenty more

You got that right, albums like ITCOTCK, Lizard, Wake of Posedion and Islands are Mellow, Soothing, Dreamy and just have a great flow and then there are albums like Red, Larks Tongue in Aspic, Starless and Bible Black and THRAK that are just downright Heavy, Loud, Outragous and certainly not for sensitive ears...That my friend, is a diverse band. :)

I noticed a lot of people naming bands that, while definitely chill-out music, aren't acoustic based, so...

Andrew Bird
Elliott Smith
Martha Wainwright
Patrick Wolf
Rufus Wainwright
Sufjan Stevens
Ted Leo
Will Johnson

I also agree with the guy who named B&S, they're an excellent band (one of my favorites, my signature is a quote from one of their songs). I'm surprised no one has mentioned Bright Eyes, not that I'd recommend him myself.

A few suggestions, some a more Jazz oriented than rock but anyway...

Charlie Hunter
Pat Metheny
Mogwai
Ken Stringfellow
Emiliana Torrini
Goldfrapp
Elysian Fields
Tim Reynolds (definately check him out for acoustic guitar chill type music)
Moloko

Dave

Add Brad Mehldau and Ray Lynch to that list too.

Dave

Down In The Park, by Gary Numan.

Joe Satriani's Album Surfing with the alien is so soothing, yet rocky. Great Chill music.:)

Waking Ashland always gets me.

Queens of the Stone Age
Kyuss
Opeth
Sunn O)))
Electric Wizard
Katatonia

radiohead-ok computer and amnesiac
john coltrane-my favorite things
miles davis-kind of blue