Cimorene

Well, haha, Supernatural's first few seasons are really buddy-adventure with the two brothers. For those seasons, there were a lot of incest shippers... I think they both had occasional het entanglements but the show (and the fandom) were really sexist, actually fairly horrible, and the female characters in their lives never lasted long.

Starting with season 4, they introduced Castiel, played by Misha Collins, and Dean/Castiel is definitely the dominant slash pairing now, although in the last three seasons they have also added some female characters in slightly less horrible ways. If you don't mind spoilers, you could read the post I wrote in season 4 when I started reading Dean/Castiel fic:

How To Read Dean/Castiel for People Who Aren't Up on SPN Canon and Don't Want to Be, OR: I Read Shipper Essays So You Don't Have To. But if you are gonna watch the first 3 seasons, I wouldn't read that essay first because it will kind of spoil most of the main plot of at least one of them I think. :D