The Reversal series
A Xander/Angel series. And written in random order (but listed here in proper storyline, such as it is). It was based on a rumor I heard about the end of the third season and the upcoming Angel. The rumor just sets things into action, though. It's definitely not the main point of the series. Unless otherwise noted, these are all PGish:
Earth-Shattering Kaboom (NC-17, I hope. This was my first smut-type thing)
Wake-Up Call (NC-17ish, I hope)
Night of the Living Crack Whores
And here's the Willow/Oz/Spike thing. "What Wasted Looks Like" (takes place a few months after "Special Guest Star") is a three-part story that should be read in the following order:
Oz
Spike
Willow (this part is NC-17. My first attempt at not-exclusively-slash smut).
The Bed follows "WWLL." Giles finds out. NC-17 again.