Torque for Android is the best low cost / free option in terms of software I can think of. Android is an OS not just a phone, you can download it and install it on a netbook if you like, but 2nd hand Android phones with bluetooth are very cheap now.
Add torque for Android ($0 for the free one to download) and a Bluetooth OBD adaptor ($10-30 from ebay) and you will have all you need I think. It logs position and loads of other stuff as you drive in a file you can download later and overlay onto google maps if you are totally geeky.
An ultimately cheaper option may be to just have an instant MPG meter (e.g. Scangauge or Scangauge E) on your dash and mount a camera to film yourself driving with a cheap digital camera from Ebay ($10-15, + $5-10 for a micro flash card). Mount it so it catches images outside the window and your gauge and then play/freeze it at your chosen points and note the figures.
I've done the filming myself idea and learned quite a few things about how I drive, plus it's useful if you have an "incident"
