We have been using DotNetBar in our Windows Forms development for several years now and it is truly amazing. Here is a brief description from the DevComponents website:
DotNetBar Suite for Windows Forms is toolbox of 61 stunning components for creating professional user interface with ease. For over 9 years DotNetBar is helping developers like you create sleek professional Windows Forms (WinForms) user interfaces with unparalleled ease.
DotNetBar Suite helps you create beautiful and functional applications without writing much code.
DotNetBar currently has over 61 controls and wraps most common controls in a wrapper layer that adds both styling and functionality improvements.
Check it out here:
Here is a link to the Applications Gallery:
We are also using the Professional Icon pack, which you can find here: