sonic_hellmouth is a Who-verse/Whedon-verse crossover ficathon run by 
clevermonikerr. Stories are due on June 1st, and signups are open now.The Sarah + Giles prompt looks particularly tasty to me. Unclear I'll be signing up, though. I need to hunt down and subdue those two linked crossovers I hinted at back in November.
Thanks to
sueworld2003 for linking this! I'd have missed it otherwise. 