Good question!
1.) For Android application development(both phones and tablets), the best resource is the developer site itself:
Android Developers
You will find all the tools, resources and training on this site.
2.) Next, for Windows Phone development, there are many sites:
- The SDK and resources can be found here :
App Hub
-
WindowsPhoneGeek
Well, then there is
CodeProject itself, where you can find many articles on application development!!!
Wish you all the best.
Sandeep.