If you haven't read Kei's "Trick or Treat", I highly recommend it. It's a wonderful read, and almost complete. It's a Xander-centric Buffy/SG1 crossover, and it presents a fully developed post-Chosen world, and absolutely wonderful characterization. It's here:
http://www.tthfanfic.com/Story-4301/Kei+Trick+Or+Treat.htm

Another fantastic Buffy/SG1 crossover is DonSample's "Misunderstandings", in which the Scoobies and SG1 end up investigating the same remote island. It's complete, and can be found here:
http://www.tthfanfic.com/Story-3244/DonSample+Misunderstandings.htm
It also has a sequel, "To Be or Not to Be", which I thought was good (but not as great), and presents the possible unexpected consequences of Dawn going through the Stargate.
http://www.tthfanfic.com/Story-6624/DonSample+To+Be+or+Not+To+Be.htm

For pure laughs, I have often re-read Kei's "A Hacking Good Time", which never fails to put a smile on my face. Another Buffy cross.
http://www.tthfanfic.com/Story-4965/Kei+A+Hacking+Good+Time.htm

And finally, while I feel really, really odd about mentioning my own fic, this one got a really good response. I recently posted a SGA/Quantum Leap crossover. It's a one-shot, but there may be a followup in the future, as the muse strikes. It's called, "In the Waiting Room", and can be found here:
http://fydyan.livejournal.com/34823.html