ScanGaugeII - I got one last week!
I can see the difference right away, so It reminds me to lift my foot a little and I get better MPG's without changing much else (Although I'm not convinced/shure that I'm actually doing as good as it says sometimes). I just ran 900+ miles this 4 day weekend and got 18.3MPG average (the traditional method of calculating), but the scangauge said I got 20MPG.
I've normally run in the 15.7 range (shorter runs of course, so not apples to apples, but I am getting better milage on the shorter runs as well - If I behave myself).