Ultimate ribbon designer : Ribbon Workbench (beta)
I think these tools are not useful anymore, since Develop 1 provides an incredible tool, packaged in a CRM 2011 solution that allows you to design ribbons using drag and drop in an amazing UI.
It is now just a matter of minutes to add a new control on a ribbon (not only buttons). See the list of supported item below:
To download this tool, just go to develop 1 website
This tool is still in beta version but is much better than any other tool I know. It promises the best for future release!