Windows Phone 7.1 Developer Tools (code named "Mango") Beta has been released on 24th May 2011. It contains a no. of new features (more than 500) including Internet Explorer 9 and Skype inbuilt in the OS. Find more about the features here. You can use this new Beta Tools to develop applications targeted to both 7.0 and 7.1 OS version. Remember that, it is still in Beta stage (i.e. development stage) and hence you can't use it for production. In this post, find the links from where you can download (both Web & Standalone/Offline installer) and install the Developer Tools 7.1 Beta release.