1) In the first one I remember a line being something like 'Dru waltzing into Slayer Central' (awesome line btw). She brought Xander to Spike for a pet. Xander had been (nearly) broken while kept by demons.
2) Dru gets to drive Xander insane this time and vamping him along the way. Xander replaces Dru in Spike's affection. Season 2 - Spike goes from New Orleans, where the Harrises were vacationing, to Sunnydale to cure Xander and Dru gets to tag along.
3) AU - Xander is hired as an actor for gay Internet porn. Spike takes Xander through his first time. And Spike is still a vampire.
Actually, I wasn't really sure how to describe this one - Spike has not been to Sunnydale so he and Xander are unknown to each other.
Many, many thanks!