If you like people encountering Superman in a universe where it's impossible, there's always my story Veritas, about 1993 Clark Kent ending up in real world post 9-11 Washington DC with a planeload of passengers from Metropolis. It's not a crossover, exactly, unless you count it as a crossover with the real world (not the MTV version). I hate to shamelessly self-promote my own story, but sometimes you can find good fics by looking at things just a little different than what you are mainly looking for.
Also, there are several Lois and Clark crossover stories over on the Twisting the hellmouth site.
