jameschick's "Quantum Xander" (
http://www.geocities.com/kiwileesa2000/jameschick) is a fascinating romp across parallel worlds with Xander finding and losing Spikes along the way. You see he made this wish, "I wish I knew how my life could have been, if I had made different choices." and we all know what happens when you make a wish in Sunnydale. It's a long tale (75 chapters) that takes a year's time to Xander though only 2 1/2 months to the Scoobies left behind and at the end he is mourning the loss of the Spike he loves but who is not the Spike of his own reality. However fear not for Jameschick has written a sequel "Quantum Xander II: In the Wake of the Curse" (65 chapters) where things become really complicated for our bois.
Jameschick is a talented writer and has written a remarkable saga. These are fun fics though there are dark moments and like Lay's potato chips, it's hard to eat (read) just one (chapter) so make sure you can block off enough time to read them.