Songs that are not traditionally scary (or even serious) but if added to a good scene makes it that much scarier.
Liiiikkkkeeee, Tiny Tim's "Through the Tulips" in Insidious
The ENTIRE Fallout 3 GNR Soundtrack like throughout the entire game. Bioshock's soundtrack too.
or even new pop songs to add a new twist to expected horror soundtracks.
Have any good ones?