Monitor mobile device locations, phone calls, text messages (SMS and gtalk), and browser activity.
This is an entire system: an Android app that runs on the mobile device (very discretely) and a PHP/MySQL back-end that logs all activity and produces a variety of reports and maps. See http://WhereGuard.com for more screenshots and details. It currently has about 80 beta-test users (not all active) and the product is very stable. We are getting almost a request a day (organically) to be a beta tester. It is Android-only right now, but the API to the server is very simple and clean so implementing an iOS version would not require a lot of effort.
Personal issues have left me without enough time to properly manage this project, but it is a solid project with a huge amount of potential.