Just started using Car Minder to keep up with scheduled maintenance and gas mileage, it's pretty dang handy. You plug your scheduled maintenance items in, then every time you fill up you put the mileage and fuel in, and it'll tell you what needs to be done based on the miles/months schedule you plugged in. It takes a bit to set up, but totally worth it. It'll graph your mileage so if you have a changing trend you can get a jump on it rather than waiting until something breaks.

Any other apps that you all use that come in handy?