Now, no longer a limited time offer.
While Microsoft allows the Windows Phone SDK and related tools to be downloaded for free, the registration fee for Windows Phone Dev Center is compulsory for those wishing to submit their apps to the Windows Phone Store. Microsoft will be hoping that by maintaining a low registration fee for Dev Center developers who have may have otherwise ignored the platform will look at creating apps for Microsoft's mobile platform.
A lowered registration fee will not be enough of an incentive in most cases, but it is one of a number of barriers Microsoft has lowered in an effort to encourage more developers on to Windows Phone. Also notable is a newly refreshed tutorial series, Windows Phone 8 Development for Absolute Beginners, and the recent release of the Windows Phone App Studio, which allows for the creation of Windows Phone apps in the browser.