If you use Windows 7 / Vista, Microsoft provides Windows Ribbon Framework - a free ribbon control that comes with the operating system (In vista, check the platform update).
The main reason why to use the Windows Ribbon Framework: It's developed by Microsoft.
This means:
Since it’s the original one, it contains ALL the features, as opposed to other free/commercial ribbon controls which always have those "not implemented" sections.
It has COMPLETE support and integration with windows 7 UI & accessibility features. Just think about touch screen support or high DPI screens compatibility.
Also, there is a managed wrapper library named
Windows Ribbon for WinForms which allows you to use the ribbon in .NET WinForms applications.