For the BtVS/AtS side of things.
About Buffy the Vampire Slayer and Angel (lbd_btvsangel)
The BtVS/Angel: the Series subset of the Little Black Dress collection, for everything Buffy and Angel. And Willow, and Giles, and Oz, and Gunn, and Wesley...
Note: stories that only cross BtVS and Angel together do not count as crossovers unless the characters involved never interacted canonically. So, a Buffy/Wesley story wouldn't count as a crossover, but a Tara/Fred one would.