A Scratch Modification (often shortened to 'mod' and not to be confused with the mathematical operation 'mod') is an edited version of the Scratch program. The purpose behind making Scratch Mods is usually the addition of new features to the original Scratch program. Because of the extra features and/or blocks, these versions are not supported by the original Scratch program and online players, and cannot be shared to scratch.mit.edu. Scratch modifications are also not allowed to use the word "Scratch" in them, with the exception of the phrase "Based on Scratch by the Lifelong Kindergarten group at MIT" or can shortened to "Based on Scratch by MIT". Additionally, you must let the source code be available. More information about the source code license can be found here.
For a list of Scratch Modifications currently known to this wiki, see this article.
For help opening a Scratch Modification, see How to Download Scratch Modifications.