Tons of great stuff mentioned in this thread so far. I've been looking for stuff like this lately myself so I'll have to check out some of these recommendations.
In addition I would highly recommend checking out:
Tenhi - Tenhi's "Kauan" is probably my favorite album ever for imagery and atmosphere. The cover is a murky picture of a fog shrouded lake surrounded by a forest. Everytime I play this album I drift away to that eerie lakeside forest. I guess I would call the genre "dark folk". Mostly acoustic guitars, violin, piano, flutes, and very melancholic vocals.
Empyrium - Similar forest atmosphere to Tenhi and Ulver's Kveldssanger especially on the album "Weiland". The entire album just feels like spending a night completely lost in dark woods with only the light of a full moon guiding you. Just an unreal atmosphere. This would be another "dark folk" band though I believe their earlier albums incorporated some black metal elements.
Harmaa - A sideproject by the members of Tenhi. Very similar to Tenhi's music, especially their stuff on "Kauan" except that its completely piano driven. Another "dark folk" band.
Negura Bunget - I'm only familiar with the album "'N Crugu Bradului" but this album is very atmospheric dark "forest" black metal. I guess the closest thing I could compare it to is Emperor's "Anthems to the Welkin at Dusk" though it's still very different from that album. There are parts on this album that sound like they were recorded in the middle of some vast Romanian valley.
There are a lot more I could mention but I'll just toss out some band names...
Agalloch
Nest
Emperor (especially Anthems to the Welkin at Dusk)
Nile (middle eastern atmosphere)
Orphaned Land (middle eastern atmosphere)
Karl Sanders (mastermind behind Nile - check out his middle eastern ambient solo album)
Devin Townsend (especially Terria and Ocean Machine)
Shape Of Despair
Amorphis (especially Tales From the Thousand Lakes and Tuonela)
Also, just about any funeral doom metal has a ton of atmosphere and imagery... Try
www.doom-metal.com if you're interested, its gotta be the most authoratative resource when it comes to learning about doom metal.