Since the release of Microsoft Dynamics CRM 2011, there have been numerous 3rd party tools, mostly free, developed for making it easier for developers, consultants, or even the power users themselves to modify the Ribbon.
I have evaluated most of them, and been disappointed with their reliability and the overall usability – some crash, some are not user-friendly, some are just taking too long to publish the changes – to a point where it is just quicker and safer to hand-craft the Ribbon XML directly.
I can recommend Ribbon Workbench by develop1: http://www.develop1.net/public/page/Ribbon-Workbench-for-Dynamics-CRM-2011.aspx.
Unlike most of other 3rd party tools, it has a WYSIWYG editor that allows you to drag and drop buttons, create new groups, insert new buttons, etc. Having this feature lets you preview your changes before committing it, saving you time from having to publish the changes and refresh the CRM page to view the changes.
Like other 3rd party tools, however, it is not user-friendly enough for power users to comfortably and confidently modify the Ribbon. The tool still requires you to have a good understanding of how the Ribbon works in Microsoft Dynamics CRM 2011, such as the concepts of Buttons, Commands, and Display and Enable Rules.
In my opinion, it is the best Ribbon editor currently available. If you disagree or think that there is a better one, please do let me know.