Supreme Shortcuts - This application allows you to pin shortcuts to to most options in the settings page on the Start Screen. There are currently 71 settings/menus/apps that you can pin and customize
LearnMango - This app has the detailed list of all the new APIs in Windows Phone OS 7.1.
You don't have to go online to check which namespace is available under which class again and again. You now have an App which has all the detials for you without going online.
It is book cum reference of new Windows Phone OS 7
