Agreed with gella, Fleet foxes are awesome for stuff from 2008.
I say any jazz music. That is, Ella Fitzgerald, Frank Sinatra, Gershwin, etc.
Goldfrapp's Seventh Tree is very acoustic.
I find Billy Bragg relaxing, doubt many others would though. lol
^^ totally forgot about chillout sessions :O
off the top of my head, one of my faves frmo a couple of yrs back
heartbeat - annie
ta douleur - camille
sweet cow - chicken lips
middle of the hill - josh pyke
listen to barling, he knows what hes talking about.
also:
Starsailor - love is here
(the whole album is great but if you want songs try poor misguided fool, alcoholic or fever)
Doves - lost souls
(again i would highly recommend the whole album but some songs are: the cedar room, the man who told everything and firesuite)