Edit: This information is outdated!
Recently, Microsoft have introduced new "Dev Center Benefits" program.
If you're a serious app development expert, as past of this program you can create developer account in the Windows Store or the Windows Phone Store for free. But then you need to publish application in one of those stores.
To get free account you need:
- Go to Dev Center Benefits
- Press "join"
- Sign in with your microsoft account (or create new one)
- Fill all necessary information.
- Create Windows Store or Windows Phone app.
Also this program give you more advantages see more information on web site.
I have published Bricks Avalanche in the Windows Store.