Official Sample Extensions:
https://developer.chrome.com/extensions/samples
Official Getting Started Chrome Tutorial:
https://developer.chrome.com/extensions/getstarted
Official Javascript Extensions API:
https://developer.chrome.com/extensions/api_index
Message Handling:
https://developer.chrome.com/extensions/messaging
Firefox Extension Getting Started Guide:
https://developer.mozilla.org/en-US/Add-ons/SDK/Tutorials/Getting_started
Extensionizr:
http://extensionizr.com/
A boilerplate generator that helps generate a manifest for you.
StackOverflow tagged questions:
http://stackoverflow.com/questions/tagged/google-chrome-extension
Lifehacker's How To Build A Chrome Extension guide:
http://lifehacker.com/5857721/how-to-build-a-chrome-extension
CSS Tricks: Building ColorPeek, a Chrome Extension:
https://css-tricks.com/colorpeek-part-2-building-first-chrome-extension/