Rathbone's recommendations include Silverlight Spy, Isolated Storage Explorer, and Metro Grid Helper:
The Windows Phone 7 team have definitely created something amazing with the Metro design styling that Windows Phone 7 comes with, but like nearly all design styles Metro looks most polished when everything is perfectly in symmetry. For Metro this means 12px spacing everywhere, run on button text, and overall a simple rule of living and dying by alignment. The tiny Nuget package MetroGridHelper (its only a single class!) serves these needs perfectly by simply overlaying a 24px x 24px grid with 12px spacing over your app to make fixing alignment issues and conforming to the Windows Phone 7 Design Guidelines dead simple.
Developers getting started should have a look through the post, and I suspect there might be one or two ideas for experienced hands as well.