It appears that "knob" is generally considered more correct now, though in old english "nob" was more common. There are many meanings, I'll show with the spelling I prefer :
knob : dial/wheel control knob : bump or protuberance knob : small amount, usually of butter knob : head of the penis nob : head of a man (archaic) nob : wealthy or upper class person (archaic)
Some people seem to think that either "nob" or "knob" are exclusively correct for the slang meaning of penis (they also disagree on whether it refers to the whole penis or just the head). It's unclear to me where the origin of this slang came from, since "nob" can either mean a person's head (archaic), which would suggest the slang came to refer to head of the penis, or "knob" can mean any protuberance.
Wiktionary seems to think "knob" is a common way to refer to a hill; perhaps this is British, I've never heard it in America.
Some weird uses :
"Nob Hill" - my guess is this common name refers to a hill where the wealthy people lived, not the fact that the hill was a knob, but I could be totally wrong about that. Some people seem to point Nob Hill at nabob but since nabob basically means the same thing as nob I don't see why you would point "Nob" at "nabob" when you could just point it at "nob".
"Hob Nob" - apparently this is completely unrelated if you believe this etymology it came from habbe nabbe
"For his nobs" in Cribbage is a funny one ; it appears to also be completely unrelated, coming from the game noddy which means simpleton, and since the knave of the same suit was important to the game it was referred to as the "knave noddy" or just "noddy" which must have become nob. (unrelated but the story of John Suckling, purported inventor of modern Cribbage, seems pretty fascinating; he was apparently a master gambler and cheater at cards who used his skill to get money beyond his station; he was involved in a plot to spring a prisoner from the tower of london, and received at least one beating at the handle of a nobleman tradgames ; ezinearticles ; wikipedia )