Growl Developer Documentation
Before you read the specific information for your language of choice, you should read our Developer's introduction to Growl, which covers the general concepts of working with Growl.
Growl is distributed under the conditions of the BSD license. The Extras are BSD licensed as well. Example applications are in the public domain.
Cocoa API documentation
Information regarding the implementation of Growl, and the Growl Application Bridge API.
Growl is written in Cocoa with Objective-C, but there support for AppleScript as well, documented at our AppleScript Support for Growl page.
For other languages, our Growl and Bindings page is what you need to look at.
Source Code for Growl
Growl source code information is available here: