Halcyon is one of the all time greatest songs Orbital has ever done, IMHO. I really enjoy the Boojah live mix w/ jon bon jovi and belinda carlisle (i'll post a link as soon as I can find the file).
As far as excellent chillout musics go......
Wir and Slow it grows - The Orb
The Art of Noise - Opus 4 (Gives Halcyon a serious run for it's money in the sheer melodic beauty department. Drum and Bass, but even still. Excellent excellent stuff. Drum and Bass Collection by Arto of Noise is one of my favorite albums on the face of the planet.)
Thrillseekers - Synathesia
Skip Raiders - Just another Day (gets a little energetic at the end, but still quite mellow for most of the song.)