I know I have mentioned the Windows Phone SDK (software development kit) a lot in the past but usually only as a reference along with other things. It really deserves a post of its own though. The Windows Phone Software Development Kit (SDK) 7.1 provides you with all of the tools that you need to develop applications and games for both Windows Phone 7.0 and Windows Phone 7.5 devices. In the past there were sometimes a couple of separate downloads required in order to make everything work together but these days one site has it all. At the AppHub you can get the Windows Phone SDK, explore the documentation, connect with the community and learn how you can publish apps and earn money.
The Windows Phone SDK includes the following
One question: When will the Windows Phone SDK be compatible with Windows 8?
Tobii, that is a good question but I don't know the answer. I know it has a high priority so I hope it will be soon.