So many great acoustic songs...
I really like the Stone Temple Pilots acoustic version of
Plush and someone mentioned Nirvana's acoustic "All Apologies" which is great too.
The Rolling Stones have several fantastic acoustic driven songs...
Wild Horses, Angie, Sister Morphine and the awesome Moonlight Mile.
Any Neil Young song is a masterpiece... my current fav is his solo performance of
Don't Let It Bring You Down... The Needle And The Damage Done,
Old Man, Deep Forbidden Lake... man I could list every song in his catologue.
James Taylor's "Fire And Rain"... "Give A Little Bit" by Supertramp is a classic... "Norwegian Wood" by The Beatles... oh, and a little ditty called "Stairway To Heaven" by Led Zepplin is pretty good in case you haven't heard of it (they've got some other good acoustic tunes like "Ramble On" or "Babe, I'm Gonna Leave You").
Of course there's great folk songs starting with Dylan, how about
Tangled Up In Blue... I like Gordon Lightfoot songs like "Sundown" or "The Wreck Of The Edmund Fitzgerald"... and Simon and Garfunkle's "Mrs Robinson" is an all time great.
Also I love
Pete Townshends acoustic work throughout the album "Tommy", the "Overture" track is one of the great peices of acoustic ever.