eccentricallypeculiar brought this up on tumblr the other day: where are all the Smallville-Marvel crossover fic?
Well, look no further:
this fic by et2brute called "Amongst The Acting Powers" is a WIP, but it's still well worth the read (and the wait).
I won't ask if you like Smallville -- if you've friended me, obviously you do (or are highly masochistic because you kept watching it anyway ;) -- but if you like the Marvel Movieverse -at all- (especially Iron Man), give this one a try. You won't regret it :)
(Disclaimers for the faint of heart: the author's self-admittedly only watched through season 4 so far, but already pretty much sees Lex's downfall as inevitable. Also, mild spoiler: this is an antivillain fic. *eg*
That said? Read it anyway! :-P)