As mentioned in the Spotify Game Music thread, the majority of Studio Ghibli's soundtrack collection is now available on Spotify, Apple Music, and other streaming services. This includes OSTs, image albums, orchestra albums and vocal collections for Nausicaa, Spirited Away, Princess Mononoke, Ponyo, Howl's Moving Castle, and pretty much all the other classic Ghibli films. The only omissions I'm aware of are Grave of the Fireflies and My Neighbors the Yamadas (though there is one orchestral arrangement for the latter on Czech Phil Studio Ghibli Symphonic Collection).
I've collected them all in two Spotify playlists:
Anime Orchestral & Instrumental Best ft Studio Ghibli - Absolute favorite mostly orchestral tracks from the many Studio Ghibli albums, including several gems from the lesser known image and arranged albums. Also includes highlights from the currently limited selection of non-Ghibli anime soundtracks available.
Studio Ghibli OSTs & Arranged Albums - A simple collection of the first track from every Ghibli-published album, as well as from a few exceptional arranged albums not published by Ghibli themselves.
Several soundtracks by none other than Yoko Kanno are available for streaming as well, mostly her Ghost in the Shell work. I'll be adding tracks from these soon. If there are any other notable anime soundtracks I've missed please update us all here.