Alright so
beer_good_foamy is one of my favorite fanfic writers... so here's 5 classics from him:
BTVS/ATVS:
Dawn Of The Dead (48 drabbles, s6, Spawn, Spuffy, Spangel... Spanythingthatmoves, really) The funniest thing ever.
BTVS:
Welcome To The Slaughterhouse (NC17). Alt "Family", 10 chapters: Tara's family aren't just rednecks, they're crazy cannibal rednecks. VERY violent Texas Chainsaw Massacre crossover. Canon pairings.
BTVS:
Well, Gosh! Season 7: How come Buffy was so powerless against the übervamps, anyway? Bible crossover. Seriously.
BTVS:
Have Yourself A Fluffy Cluffy Buffy Christmas. Post-series: Buffy and her husband celebrate a very special christmas. Pairing: B/?
ATS:
Testament. Post-series, 3 chapters so far: Faith deals with Wesley's death (WARNING: WIP)