When Microsoft announced Windows Phone 8 back in June at its developers event, they also said that an SDK would become available in late Summer. While Microsoft has yet to officially release the SDK, a build of the platform has found its way out on to the Internet.
The SDK requires that you have Windows 8 Release Preview installed and the download comes in at 1.35GB. The build has a tagged date of July 19th, meaning it was fresh off the compiler when it hit the web.
The image above provides a bit more information and there are a few other documents (help files) floating around with the SDK. If you do manage to find the SDK, install the software with caution as it is an unreleased product and has the potential to bork up your install or even you phone if you connect it to the platform.