Scratch

What is a Scratch mod?

A Scratch mod (short for "modification") is a derivative of Scratch based on the Scratch source code. The license that the source code is released with lets anyone create a Scratch mod and share it with the world, on a few conditions:

The Scratch Source Code License
The Scratch source code license allows you to distribute derivative works based on the Scratch source code for non-commercial uses subject to the following restrictions:
you cannot use the word "Scratch" to refer to derivative works (except in the phrase "Based on Scratch from the MIT Media Laboratory"),
you cannot use the Scratch logo or the official (copyrighted) Scratch cat in derivative works,
you cannot implement the ability to upload projects to any MIT Scratch website (currently, http://scratch.mit.edu),
copies or derivative works must retain the Scratch copyright notice and license, and
you must make the source code for derivative works available.

Examples of popular Scratch mods include Panther and BYOB. Scratch 1.4 is written in Smalltalk and based on a Squeak VM. Don't worry if you don't know what either of these things mean -- this tutorial by Jens will help you get started. If you're serious about modding Scratch, there are lots of resources in the Advanced Topics forum to help you out.

One last thing: if you didn't read the list above, one of the important stipulations of using the Scratch source code is that your own mod must be free and open-source and use the same license that Scratch does. This extends to allowing other users to modify and use your source code -- it's all part of the cycle of open-source code.

Note: some people use the word "mod" to refer to Scratch Community Moderators.