Winter's Storm by greyangel
Spike is the head of the LA branch mob, and he takes Buffy hostage. For a lot of the fic, Spike's really horrible to her, but it's a great story. I have a copy of it because I don't think it's on the web anymore.
Mad World series by pepperlandgirl
Also not on the web, but I do have a copy. Spike is a crooked cop and Buffy is a prostitute.
The Baby series by Mrs. Muir
Summary: The series begins during season six and continues through the next decade while Buffy and Spike work their way through love and life.
Ain't No Sunshine and Wheel in the Sky by Mrs. Muir here on SR