1.) The WP7 AppHub http://bit.ly/g4cHgG
This is more then just the place to buy or sell your WP7 applications. From the education tab (the link I shared above) you can get code samples for anything from simple silverlight apps to 2-D and 3-D XNA games.
2.) Windows Phone 7 Development for Absolute Beginners http://bit.ly/hWemZN
If you are start off from scratch (either a beginner programmer or transferring from another language), this is the place for you. This is step-by-step for the absolute beginner
3.) Tim Heuer’s Blog http://bit.ly/f5Cct5
Tim Heuer not only talks on all things silverlight, but also dabbles in the WP7 phone from time to time with some really great insight.
4.) The UI and Interaction Guide for WP7 http://bit.ly/dG2Tex
I know, I know, as a developer you usually stay away form UI stuff… but for WP7 you really need to understand this stuff. These guides will bring you up to speed.
5.) The Windows Phone 7 Training Course http://bit.ly/fWZPwP
For those that are more than beginners but still want a step by step approach to building an WP7 application, the hands-on-labs here will get you going.
6.) Charles Petzond’s Free Programming Windows Phone 7 book
What else do I need to say… it’s a free book
7.) Shawn Wildermuth’s Developing for the Windows Phone 7 Series http://bit.ly/fUBo9i
Shawn does a great hands on course on Windows Phone 7. This series scratches the surface on what his class offers.
8.) Windows Phone 7 Sample apps http://bit.ly/gZS33V
A great list of sample applications for you to learn from from Compiled Experience blog.
9.) Windows Phone 7 and Azure http://bit.ly/fXkcBd
WP7 and Azure, a love story…. no, just kidding.. but it is a great set of post on how to use WP7 with Azure Cloud Services with Bart Merchtem
10.) Windows Phone 7 Patterns and Practices http://bit.ly/e2w1sK
The definitive guide to the how and the why on Windows Phone 7
Well, there it is, my Top 10 resources for WP7. To tell you the truth, this list could be 100 or more strong but we all know people stop reading at 10 
Hope that helps
Daniel – The Sociable Geek