Willow & Xander
Willow & Xander were always best friends, since they were both little. Unfortunely Willow for a long time had a crush (a big) on Xander but Xander was very oblivious and never recognized her feelings because he was too interested in Buffy since she moved to Sunnydale. Willow always tried a way to get his attention. Having their first feelings in Season 2 opener "When she was bad." They were eating ice-cream and had intimiate feelings and were about to kiss when a vamp attacked and was staked by Buffy; Buffy making her entrance back home from a summer vacation with her father in Los Angeles. That moment was lost forever.
Then during their senior year near Homecoming, they were picking out clothes to wear, giving advice to eachother. At the time, they were both with steady boyfriends and girlfriends. When Willow tried on this one dress which she looked beautiful in, it caught Xander's eye. Xander and Willow kissed but then ended it cause they knew it was wrong and they couldn't have feelings. Then Spike (Lover's Walk) kidnapped Wil and Xander to do a love spell to bring Dru back in his life. They were frightned and afraid of dieing because they were trapped in the basement of the old factory. They started making out when Cordy and Oz came in to the rescue to save them, but they wond up seeing them kissing.
This tore Xander and Cordelia. Cordelia was depressed and heart broken over the incident. Xande and Cordy never went back together though they did become "sort of" friends again. On the other hand, Willow and Oz got back together. Willow and Xander's feelings may still be there about eachother but it never happened again and their relationship is entirely over for now, maybe temporially. Hey ya never know!
The Evil Will and Xander
In Season 3's "The Wish" Cordelia made a spell that Buffy never came to Sunnydale.
This wish actually came true due to Anya, a demon. (an ex-demon now) In the wish, Willow and Xander were the head vampires for the Master. They were killers and the best vampires of the Master's group. They were also vampire lovers throughout the whole espiode. It was very cool (in my opinon) seeing them vampires and lovers.