I'm doing a paper on Slash Porn for my "From Pulp to Porn" class. Slash porn, for those of you not in the know, is a form of written erotica. I only started reading it about a year and half ago. It is written predominantly by women for women and it was started in the 1970's with Kirk and Spock stories (Kirk/Spock or K/S which is where the name "slash" came from, though I'm sure we all get the double entendre). Women started writing stories wherein Kirk and Spock were lovers. It expanded to Starsky and Hutch and is now a major form of fan fiction. There is now tons of it and if you want a good intro to it go to the Good Girl link on my blog and go to Back Issues and scroll down until you get to Volume 2 "all things sexual" and then to the article "How Slash Saved Me"
It's almost always homoerotic, and you can find stuff written on many shows - Due South has Ray and Fraser going at it and you can find crossovers like Highlander and Xfiles and shit. There's a lot of badly written stuff out there, you can find quite a bit online, though many many fanzines are dedicated to specifically slash. The best stores are obviously the ones which really capture the tone of the series they are writting about and make is seem like a plausible story line.
Since I know I have many Buffy fans out there, if you're interested in a little Spike on Xander action, please check out one of my faves truthordare it's called Truth or Dare. It's long, but if you've got some time on your hands...