flowspuffy beat me to it with her crossover list, which looks terrific. (I was internetless yesterday, so am late to the party). Anyway you can never have enough crossover recs, so here's another five that I hope you enjoy.
A Girl Made of Sky by Doyle is a BTVS crossover with Neil Gaiman’s Sandman and features Dawn and the Cheeseman. I can’t say anything about this other than I wish I’d written it!
Neverland by tabaqui has Spike rescued from Giles’ bathtub by a boy who can fly. This is a Peter Pan xover, with Xander as a lost boy and Spike as a lost vampire. This is well worth a read.
Spell Check by speakertocustomers is a bit of very short demented black humour that finds Willow assembling her ingredients for the resurrection spell. Narnia will never be the same again…
No Power on Earth by redikki is a piece of BTVS/Supernatural heaven. There are some things you never talk about and for Xander and Dean, the Fabulous Ladies Night Club is one of those things… This is short and lovely and has a subtext a mile long.
Finally, probably my favourite crossover of all time. I think I recced it here a few years back, but it does no harm to share the love again, because it’s so good.
When Hellmouths Collide by Kimberly Rector and Martha Wilson is a BTVS crossover with Xena and Hercules. I avoided this story for the longest time because I haven't seen the other two series, but in the end it doesn't really matter - the authors do all the heavy lifting so you don't have to. This is an ensemble fic in which The Scoobies get swallowed by a giant snake and spit out in another world. There’s so much to love about this story, I don’t know where to start, but Giles’ snarky internal commentary as he contemplates the rest of his life in this strange new place is a joy. This is a huge story, but there is a zip file for ease of download.