I really like this one.
Black Silk by Nicky
http://spikeluver.com/SpuffyRealm/viewstory.php?sid=4704&warning=NC-17Summary: COMPLETE 2 Chapters. At a sleep-over, 15-year-old Buffy is dared to sneak into Cordy's step-brother's bedroom and steal a pair of boxers from his draw – but Spike comes home early, and Buffy gets a lot more than she bargained for. (Warning: underage sex.)
I don't know how I could forget this one.
Tainted Innocence by Peroxide_dreams
http://spikeluver.com/SpuffyRealm/viewstory.php?sid=24178&warning=NC-17Summary: **Winner for Best Fantasy Readers Choice at the Spark and Burn Awards **After being dumped by her boyfriend, Angel, Buffy is forced to spend the summer with her mother's boss, Cordelia Chase. Just when things start to look up, Cordelia's playboy husband Spike turns her world upside down.