OpenPaths is a mobile application that allows you to log your location in a way that minimally impacts your phone’s battery life, and keeps that information secure (supposedly out of reach of law enforcement w/ zero knowledge encryption) while still allowing you to do interesting things with it, via an OAuth API.
A great visualization of one person’s location, as tracked for 6 months by their cell phone carrier. The person is Malte Spitz, a Green Party politician in Germany. He fought a legal battle with Deutsche Telekom to obtain their records of his movements. This kind of data is collected on essentially everyone with a cell phone, and last year in the US carrier Sprint provided it more than 8,000,000 times to US law enforcement via a convenient online API. Many of them also sell the data in aggregate for marketing purposes.