Not completely acoustic, but one of the best mellow songs of all time is Pink Floyd's "Wish You Were Here". Then there's King Crimson's "I Talk To The Wind" (some amazing instrumental work in that song, hell, that entire album).
And another band you CANNOT FUCKING PASS UP if you're into the mellow/acoustic stuff is Neutral Milk Hotel. In The Aeroplane Over The Sea is by far one of the most life-changing things you will ever hear if you can get into it and appreciate it. And yes, it did deserve it's own little paragraph.
Sufjan Stevens has done some good stuff, in particular the "Illinoise" album.
So yeah, check this stuff out.