An excited post on idolninja.com outlines plans for Volition to release details for Saints Row: The Third that will allow the creation of mod tools for the open-world action sequel (thanks nin). This will also include information for Saints Row 2, but more significantly, is said to ne a "test run" for similar support for the upcoming Saints Row IV. The post has details on how this all came to pass, and here's an excerpt describing exactly what it means:

Jeff Thompson, the Studio Director of Programming at Volition, is putting together a package for us containing documentation, file formats, tools, and more on the Saints Row: The Third engine. Myself, Minimaul, and gibbed will be working closely with Jeff over the coming months to use this information to create a robust set of modding tools to supplement our existing ones and creating what is essentially a full sdk for the game! But, that’s not all! Saints Row: The Third is only the beginning! Jeff has confirmed that he will also be digging up all the Saints Row 2 info that he can so we can do the same for it as well, and that this undertaking is actually a “test run” for Saints Row IV.

So, what does this actually mean to the average Saints Row player? In simple terms, the modding community will finally be able to create and offer new clothing, new vehicles, new guns, new npcs, new missions, and even new world geometry which will greatly extend the life of the series and enhance the Saints Row games in ways we never even dared to dream of.