There's been some great recommendations so far.
Anyway, it'd help if you gave more examples of the sort of thing you like, but going from Harold and Maude alone, I can recommend;
Wes Anderson - the earlier the better - Bottle Rocket and especially Rushmore, which I found very similar to Harold and Maude - always uses great soundtracks
Also try Hal Ashby's other films. They're not as quirky as Harold and Maude but The Last Detail is a masterpiece and Bound for Glory is good. Oh yeah and Being There, which is more left-field too.
More great American films from the 70s:
Anything Altman - The Long Goodbye, A Wedding, McCabe and Mrs Miller (great soundtrack too), California Split - Brewster McCloud - similar film with Bud Cort and extremely quirky, obviously a big influence on Magnolia.
-Punishment Park
-Two-Lane Blacktop