Some of my favorites have already been mentiond, i.e. many spaghetti western and blaxploitation film soundtracks. Yes, Morricone hits the nail quite often in spaghettis, but I also like his soundtrack for Gillo Pontecorvo's Burn! (Quemada). Also a great film, Brando puts up a fantastic if not heralded act.
I agree with all the posters regarding Zbigniew Preisner's music for Kieslowski's films, but I'd say the soundtracks for Three Colors trilogy and Decalogue are better than Veronica. There is another excellent Polish composer, Krzysztof Komeda, who deserves a mention here.
How about Gato Barbieri's score for Ultimo tango a Parigi (Last Tango in Paris)? Fantastic.
John Williams' score for The Long Goodbuy, another fantastic performance with the lead tune permeating the film in many variations.
Ascenseur Pour L'Echafaud by Miles Davis? Nuff said.
Dead Man Walking? Brilliant. Wedder would make a fine Qawwali singer, but Nusrat Fateh Ali Khan makes a statement who the Big Daddy is.
Of the Blaxploitation films not mentioned here, Across the 110th street deserves a place if only for the fantastic Bobby Womack's tune.
I'm not sure if Harold and Maude qualifies, but the selection of Cat Stevens' songs is one of my all-time favorites.
Of course there's many more, from numerous brilliant scores for Italian horror flicks (Giallo) to many great jazz soundtracks for French 50s and 60s movies to incredible music written for difficult to find Brazilian New Wave films from the 60s and Huangmei opera based sountracks for early Shaw Brothers productions. I am not into Chinese music, but one has to admire the invested detail to make a transition from folk music and dance to kung fu movies.
Of yes, here's a couple of more nice soundtracks. Orfeu Negro by Marcel Camus features some incredible music by Antonio Carlos Jobim and Luiz Bonfa.
Angelo Badalamenti's scores for David Lynch films are all fantastic. Some have already been mentioned, but I also like the soundtrack for Lost Highway including David Bowie's and Trent Reznor's songs among others.
Cat People by Giorgio Moroder. Excellent soundtrack.
John Carpenter has written some brilliant music in addition to directing his films.
Soundtrack for Mishima: A Life in Four Chapters by Philip Glass? Piece of art.
One off the bitten track: OST for Macedonian film Before the Rain by Anastasia. Check it out on Grooveshark if you cannot find the album.