SP SIN is a free solution (available on CodePlex) created by Bjørn Furuknap
SP SIN (from SharePoint Script/Style INjector) solves an age-old problem for developers and administrators, how to add script and style sheet resources to SharePoint without modifying master pages.
This problem is more complex than it seems because SharePoint allows and encourages changes to systems that are in production. How can you ensure that the changes you make work before deploying them to all users? How can you create versions of script and style sheet resources so that you can maintain backwards compatibility with previous solutions?
SP SIN solves these problems by allowing site owners, or those granted permissions, to inject script or style sheet resources into pages without touching any of the pages. All of this is accomplished using SharePoint best practices and recommended approaches only.