- xda-developers
A very awesome and very active place for anything smart smart phone related. -
kernelnewbies
Generally good resource for starting out learning linux kernel
(Which is what the android is built off of) - Building Android Kernel
Android Kernel Source - Linux kernel
Online guide to driver development among many other related topics.
Update
For android source code you can check out one of the following…
I personally dig this chrome plugin for browsing android source code.