This is a free tool, GUI based, that can be used to convert a normal installation of a Win32/.NET application into an AppX package, ready for upload in the Windows Store, or testing on your dev machine.

The main advantages of this tool over the command line tool built by Microsoft are highlighted at the end of the linked article.

We've got some feedback from the Desktop Bridge team at Microsoft, but we appreciate even more the feedback from direct users, so don't hold back.

Please note that this tool is completely separate from our (I work at the company that built it) commercial product present on the same website, that product is not the subject of this thread.