sapiens

To my way of thinking, there's a difference between a soundtrack with songs written for the movie and a soundtrack-compilation. I like both:

Original Soundtracks
Elevator to the Gallows (Miles Davis)
Anatomy of a Murder (Duke Ellington)
The Piano (Michael Nyman)
The Buena Vista Social Club (maybe not original)
Akira (Geinoh Yamashirogumi. A bit disjointed, but really good).
The Muppet Movie (Someone, Derwood I think, already mentioned this. Definitely good and definitely fun).

Compilations
32 Short Films About Glenn Gould (Great movie and great compilation of classical piano)
All the Mornings of the World (The music of Saint Colombe and his student Marin Marais. Really good!)
Good Morning Vietnam (Motown!)
The Big Chill (Vol. 1 & 2) (Can't get enough Motown!)
O Brother Where Art Thou? (Mmm... Bluegrass....
So I Married an Axe Murderer (Cheesy early 90s pop)