I have this one:
Sale! 6 x 12 15V Glass Solar Panel w/Wire Tabs & Frame-Electronic Goldmine
I put it on the back of the sunvisor and wired it to the hot lead of the dome light. It doesn't directly increase fuel economy, but it makes me feel better about coasting home at night. I know that the partly-drained battery will recharge the next morning. I park facing the morning sun! The panel will also recharge a dead battery enough to start an engine. Of course it'll take a few hours, but it will do it.
