Simple Latitude is like its now long-gone Google namesake, an app which tracks where you are and where you've been and can share that information with your friends and family making it ideal for keeping tabs on granny while you're at work.
It works in the foreground, under lock screen and in background mode.
You can either create an anonymous account or if you want to share with friends use your MS account. You can display the location of your friends on the map. The app updates your location every minute which is somewhat more frequently than I remember Google Latitude doing.
Got a Windows Phone? Wanna spy on your friends? Get this app!