In California you can have your rear windows tinted as dark as you like, so long as you can still see out of them.

You cannot have your front side windows, or your windshield tinted, at any level what so ever.

There have been rumors that if you have some kind of skin disease and get a doctors note, you can then get the tint added to the front -- but even then you can be cited for it, you'll just have to take it up with the judge/ref and have it cleared.