KludgeHack

Free controls for Windows.Forms

The Krypton Toolkit is a free package from the Component Factory that offers several improvements to the Windows Forms providing the Office 2007 palette to all the components you use. You can easily customize so it uses your own color palette.

As any professional tool, this comes with full documentation and some samples.

You can download the latest version here. Also, keep checking their blog for news.

If you want to customize further you can have a look at their paid professional package: Krypton Ribbon. Although its not free (99$), its definitely worth the money.