Honda CRX HF - got 50+ mpg in 1989 :)
92 - 96 Civics can get you low 40's if you go easy on the pedal.

Another good option is late 90's to early 00's saturns. They get 30 plus and can be found for super cheap. Their resale value is awful, but they have good little motors in them.

A good read along the lines of this thread, at one of my favorite blogs:

http://www.carlustblog.com/2008/04/car-lust--geo-m.html

I had found this site a long time ago,
people put real milage reports on here:

http://www.fueleconomy.gov/feg/drive.shtml