With A Star by Don Sample is a nice quiet BtVS crossover; the crossover is with Lois McMaster Bujold's Vorskosigan novels, and it isn't a time travel story, just a (very) post BtVS Season 7 story in which Miles stumbles across an ancient mystery while honeymooning beside the Sunnydale Lagoon. Don has also written Harry Potter and the Key of Dagon, one of the few BtVS / HP crossovers that doesn't make me cringe with embarassment.
Stupid Portal by Tara Keezer is the best long BtVS / Star Trek crossover I've seen, and was an important influence when I was writing Omphalos. The best BtVS / Trek crossover I've seen, fulls stop, is Jennifer Oksana's Real, More or Less, also an important source for my story.
Finally, rozk herself has written some excellent stories, mostly slash, that can be found at
I particularly recommend Decent Interval, Decent Interval, and the Faith By Night series.