Here's some recommendations for free ambient music. Would appreciate some return suggestions, if you've got any.
Aidan Baker & Ben Fleury-Steiner - Second Week of the Second Month - I've read people on here praise Nadja, so may as well start with these guys from Canada... two guitarists make beatless drifting with all sounds sourced from their guitars. It's described as "drone" but there's lots of bright chiming sounds and so on.
Chris Herbert live set (click the Audiomulch Sound Out button) One big file, but worth it. Chris released his first album on reasonably awesome label Kranky.
Loscil - Stases Another Kranky signee, but for this release the Canadian producer takes his released tracks and ditches all the bits that change.

Heh. Deep drone. Emperor's New Clothes. Who can say? Any of us, I guess.
SummerTour Remixes 1 & 2 - Japanese-born NY resident Sawako did field recordings while on tour and offered them up to randoms to remix. There are some beaty tracks in there, a few guitar bits, but mostly it sounds like just processed field recordings. Marihiko Hara and Steinbrüchel both turn out particularly awesome bits. Anything on Anticipate's worth checking out, although this free releases aren't 100% indicative of what the label's output is usually like.
The Noise & The City - One person from each of 30 cities around the world made field recordings from somewhere in their hometown and made music from them. Some stuff is glicky and beaty, e.g. Manchester's Remote Viewer, Sydney's Roboneko; other bits surprisingly noisy and intense, e.g. Americans Greg Davis and Joshua Treble. It's a bit hit and miss, but definitely a great concept and some really good results.
Jodi Cave - Absent British electro-acoustic composer type does a small EP of rattling niceness. Also recommended is his single track 'Flotsam & Jetsam' which is on
Fat Cat Records' demos page and also on a compilation from the crazily named
Frozen Elephants Music net label. His tracks tend to be a combination of sort of scuffed and scraped textures (contact mics?) and processed chiming guitar harmonics.
^ Pretty much everything else on
12k's term sub-/net-label is worth a listen if you're happy with some abrasive bits and some tension.
Pablo Verón - Nieves - Lots of interesting stuff on Argentinian net label Naturalmedia, which I can't link you to directly due to the joys of Flash. The first track is the most difficult listen, so don't be put off if that's a bit much for you. It's definitely a bit more in the glitch school of ambience - skittering crackly rhythms right up in the high end and soft, deep chords punctuated by kick drums. Roughly in the territory of early Pole, Oval and Shuttle 358.
I could keep going and going and going, but am conscious this isn't that commonly discussed here. Let me know if you're interested in more. And, yeah, please share your own suggestions.