What on earth has built up on the inside of my windshield, and what do I use to clean it. Seems like standard glass cleaners are unable to get rid of this layer of filmy crap on mine. Every time I get back in the sun at a certain angle, I can still see a smoky residue.

What do you guys use?

It's just a residue left by the off-gassing of all the plastics used in the interior......it easily gets to 140 degrees in there if the car sits in the sun with the windows closed.
Anyway, I just use the standard stuff like Windex, but if that doesn't work try vinegar and water.
Dave