UI Guidelines
The guidelines also include older Windows Mobile, Windows CE, etc. Some are mixed so I just put them all together to make it easier to keep track of. Do note there are still new Mobile devices being launched, mostly in speciality industries, but it happens. You might need these guidelines. Really.
UX Guidelines for Metro Style App Development from the MS Blend group.
User Experience Design Guidelines for Windows Phone, meaning Windows Phone 7, from Microsoft. Seems to be a regularly-updated kbase, so may continue to evolve.
UI Design & Interaction Guide for Windows Phone 7 v 2.0 PDF, from Microsoft.
Stencils & Templates
A mixture of Windows Phone 7 and 8, with some RT tablet and even Windows 8 Pro thrown in. Many are not easily differentiated, so I didn't bother organizing at all. Read carefully if your needs are very specific.
Design Templates for Windows Phone 7 from Microsoft. Includes 28 layered Photoshop template files and the Segoe system font. Sorry it's a dev forum post, but MS seems to not want anyone to know it exists. (Photoshop)
Microsoft Windows Phone 7 wireframe stencil for Omnigraffle from MarlinMobile (OmniGraffle)
Microsoft Surface RT .psd and .ai templates from Eric K (Illustrator, Photoshop)
WP7 Controls Not tested, from Balsamiq section FWIW
WP7 Bing Screens Not tested, from Balsamiq section FWIW
WP7 Panorama Not tested, from Balsamiq section FWIW
Microsoft Windows Phone 7 wireframe stencil via Graffletopia. Note it is full size so will be very big on letter sized paper. Use larger paper sizes. (OmniGraffle)
WP7 Passcode Lockscreen From an iOS (non-retina) project so scale may be odd (PNG)
[[http://365psd.com/day/2-136/|]Windows Phone 7 PSD] (PSD)
WP7 Start Screen (PSD)
Audio Player Windows Phone 7 (Fireworks)
HTC Mondrian Concept (PSD)
Microsoft Windows Phone 7 Emulator Not an emulator. (PSD)