These are just the ones that come to mind right now: oakland county, providence, dupage, king, fairfax, essex, norfolk, san bernardino, los angeles, orange, spokane, sedgwick, sonoma, fresno, queens, cook, wayne, almeda, sacramento, san francisco, oklahoma... Don’t ask why these are the ones that come to mind, but I would bet I know 70-80% from books and movies, the rest from news articles.

Knowledge of counties are somewhat of a function of the state. In some states, counties (or their equivalents like Parishes in Louisiana) are fairly powerful. In others, they're almost meaningless. I live in Massachusetts and I could name maybe 3 or 4 counties--and that's because I've lived in two of them.

I don't think MA has much more than that :)

It actually has 14 apparently.