While there isn't an "official" Fitbit Windows Phone app, Fitbit Companion is nice option for the Fitbit users in the house and a step in the right direction by giving you access to most your Fitbit statistics. For those not familiar, Fitbit is a small fitness monitoring device that tracks your activities and sleep patterns.
Along with the monitoring device, Fitbit also offers a fairly extensive web based tracking app that syncs with the data from your Fitbit device and allows you to enter meals, activity and track your weight. The Fitbit Companion app taps into the web based app and delivers most of your Fitbit stats to your Windows Phone. The app uses the Fitbit API and doesn't share your personal information outside of the app.