Voracity writes great SPN/BTVS crossovers.
http://voracity.e-fic.com/spnindex.html

She also writes SPN/Muti-universe crossovers:

Family Redo
http://voracity.e-fic.com/xover/hpbuffyspn.html

P. F. N.
http://voracity.e-fic.com/xover/psychofriends.html

Strange Clans
http://voracity.e-fic.com/xover/demondons1.html

Project: Future Perfect
http://voracity.e-fic.com/xover/futureperfect1.html

Family Staffs
http://voracity.e-fic.com/xover/noblefamilies1.html

Demon Criminal Investigative Services
http://voracity.e-fic.com/xover/dcis1.html

Also I'd reccomend Twisting the Hellmouth for BtVS/SPN crossovers
http://www.tthfanfic.com/Category-1-30/Supernatural.htm

storm