Logging my car's mileage online
For the past two months I've been logging my mileage on MyMileMarker.com and am happy with the results it provides (given the amount of effort on my part). Every time I fill up I just take out my phone (they have a mobile version) and log the current odometer mileage, gallons required to fill up the tank, and price per gallon. I'd always calculated my MPG in my head after filling up but I find it a lot more interesting to have the data recorded and displayed in pretty graphs (see below).
Although my car's mileage still sucks, it was nice to see it go up by about 2mpg after changing my air filter -- a $12 part that took me about five minutes to replace.
Of course there's no way for them to verify that the numbers people enter are accurate, but I'd really like to see how my car performs compared to other Volvo V40s out there. Maybe they're already working on this, but I haven't seen them push any updates to the site in quite a while now.