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.
Information regarding the implementation of Growl, and the Growl Application Bridge APIs.
Growl is written in Objective-C, but there are bindings for several other languages.
A Growl user has implemented our network protocol for Delphi/Pascal. Keep in mind that, since this a third-party product, we do not support it. You will need to contact that project's maintainers if you want support.
Source for Growl, examples, and language bindings.
Direct access to the Growl Source repository.