I second porsche's explanation. I would also note that in porsche's last example ("I haven't known anyone who can make good pizza at home"), it's more common in American usage to say "I have never known," an expression that works the same way grammatically.

It's not uncommon to see this tense used to refer to one's state of mind during a period in the past, as in: "There have been times when I haven't known what to do." The simple past is better here ("There were times when I didn't know what to do.").

An interesting case is when you're using the verb to mean "to know [a person]." It's perfectly correct to say, for example, "I haven't known you long enough to guess your political views." I think this is unique to English, as other languages I know have separate verbs for "to know [a fact]" and "to know [a person]."