I have one, there could be more but this is one that I thought of right away, its pretty short.  It's by Trista and its called 1977 and I can't remember currently where I read it.  Here I think.

There's an excellent one in which Spike and Buffy are both send back to 1970's NYC (to fetch a book or something? I forget) She feels obligated to prevent evil Spike from killing Nickie -even though she knows that she could be changing everything, including their relationship.  Much angst and worry.  I can't remember the title or author, though (sorry), but you might want to check on the LJ community Seeking Spike (http://community.livejournal.com/seeking_spike/)  Someone always knows the answers there. :)