I can definitely recommend several SPN/BtVS crossovers by virtualpersonal. Here is the link to her SPN/BtVS crossover listing with links.

http://www.livejournal.com/tools/memories.bml?user=virtualpersonal&keyword=Fic+-+Crossover+SPN+%26+BtVS&filter=all

I can vouch for An Unholy Union and its sequel When Darkness Falls which feature Spike as the crossover character.

There is also several other interesting pairings including Buffy and some of the other scoobies.