Vent Visors? like these?
I've got the stick-on type... AVS brand (p/n 94249) that I bought through Amazon for ~$50 and have held up just fine under the FL sun for nearly 2 years now. They work OK for allowing me to crack open the windows while @ work, but don't really serve any other purpose.
Furthermore, they make it a bit of a PITA when I want to remove the doors as I can't grab the door by the window frame, hang it off my shoulder, or hang the door in my garage by the window frame.