Metro Design for WP7 — Jeff Wilcox provides some basic UI guidelines for devs.
Silverlight Unit Testing Framework for Windows Phone 7 — Jeff Wilcox’s adaptation of his Silverlight Unit Test Framework for the WP7 emulator.
Unit testing cheat sheet — Exellent quick overview of the basics of using the Silverlight Unit Testing Framework for WP7.
Caliburn.Micro — Opinionated MVVM framework for Windows Phone, Silverlight and WPF.
Microsoft Advertising SDK — Get the tools to integrate display ads into a WP7 app.
Screenshot tool for the WP7 emulator – Makes it easier to grab screens for the Marketplace.