...Jonathan and Martha Kent weren't Clark Kent's "foster parents."
(I should mention I'd been about 3/4 of the way through writing this entry when I clicked the wrong thing in Netscape and lost what I'd written, so this'll end up being shorter than originally planned).
Foster Parents typically take temporary custody of a child which the state has taken away from its biological parents for various reasons. Most foster children are not legally free to be adopted, although some may end up being legally free if the bio-parents' rights are terminated (which usually takes a pretty extreme situation, such as if the parents are going to be in jail for a long time, or if the child would be in danger of being harmed physically or psychologically if they were returned to their bio-parents).
Since the Kents were pretty much consistently shown as adopting Clark (from the orphange they brought him to after they found him in a rocket), they were his adoptive parents.
Same thing goes for Linda Lee Danvers, aka Supergirl. Fred and Edna (that was her name, right?) Danvers adopted her. Yes, Kara Zor-El's parents were later found to be alive, so you could possibly argue that while the Danvers were Linda's adoptive parents, they were Kara's foster parents... except that there's no legal tie between the Danvers and Supergirl.
So, where does that leave people like Oliver Queen and Bruce Wayne, who had Roy Harper and Dick Grayson named as their wards? Dictionary.com lists one definition of ward as someone who's legally placed under the care of a guardian by the court. Now, that pretty much seems to me to be the definition of foster care, save that a ward probably won't be returned to their biological parents (whatever the circumstances there are), but also isn't necessarily legally free to be adopted by their guardian.
Which still pretty much makes Oliver and Bruce (by today's standards, anyway) foster parents. Technically, I suppose they aren't, since neither of them ever appeared to be licensed, and basically they volunteered to take guardianship of the boy in question.
Were there any real foster parents in comics? Well, Ben and May Parker probably should be considered Peter's foster parents. There are quite a few foster parents who became licensed so they could care for the child of a blood relative (it could be their grandchild, or a niece or nephew, or even a young cousin). Actually, in most cases, the court prefers to place a child with a relative whenever possible.
I don't recall reading anywhere that Peter Parker's parents named Ben and May his godparents, which I believe would've made them his legal guardians upon Peter's parents deaths... so we have to assume that they became licensed so Peter could stay with them.
The Original Human Torch's relationship to Toro is legally unexplainable to me... I mean, the Torch was an android, and wouldn't have had any legal right to make Toro his ward or foster child or anything... but it was a simpler time then, wasn't it?
I can't think of any others off the top of my head... but if you can think of other comics characters raised by someone who wasn't a blood relative, let me know!
(By the way, I don't think anyone could possibly consider Bova to be Pietro and Wanda's foster parent... because an evolved cow wouldn't have had any legal right to be such anywhere, except maybe in Wundagore, assuming the High Evolutionary cared enough to make any rules concerning that).