Gas costs sure have risen lately, at least in the US. With many drivers are seeing prices well over $4 per gallon of gasoline, the time has come to do something about it. The FuelFrog website, as well as its iOS app or Twitter, can help with this. It helps you keep track of how far you've gone, and how much gas you've used, as well as trends for gas prices in your area. Of course, there's also the FuelFrog API which, despite some security concerns, lets you integrate that data into just about anything.
The API doesn't currently require a key, but does require the username and password of a user. As a note, this is transmitted in plaintext, so if you have a FuelFrog account, please don't use a user/pass combination you use elsewhere.It is REST-based, using JSON for all formatting with no other options currently. It has the ability to show recent fillups, and add a new fillup as data, which they call a fuel Resource. All in all, it offers a neat service, but seemingly at a price: information security.
The API as it is written uses passwords in plaintext, so anyone with any sort of packet sniffer can see what username and password you have. In addition, they would be able to see where you've been, and who you are thanks to the Twitter Integration. This bothers me a lot from a privacy standpoint, as it could allow for some pretty serious stalking if a nefarious person decided to use this API for ill purposes. As a systems administrator and programmer, this lack of attention to security really grinds my gears.
However, if the usefulness outweighs these concerns for you, I might suggest not using the Twitter integration, and using a user/pass you don't use anywhere else, so it couldn't be linked to you. If you do this, it could be a rather useful service for you.