"Ookii.Dialogs is a class library for .Net applications providing several common dialogs. Included are classes for task dialogs, credential dialogs, progress dialogs, input dialogs, and common file dialogs.
This package contains two class libraries: Ookii.Dialogs.dll for use with Windows Forms, and Ookii.Dialogs.Wpf.dll for use with Windows Presentation Foundation. The classes inside are pretty much identical; only the input dialog is not available for WPF. Some additional utility classes for Windows Forms are provided that are not available for WPF, see below for details.
The included sample applications Ookii.Dialogs.Sample.exe and Ookii.Dialogs.Sample.Wpf.exe demonstrate the dialogs for Windows Forms and WPF respectively. View the source of these applications to see how to use the dialogs.
Full reference documentation for the class library is available in the online and in the help file included with the download.
This is a pretty cool looking collection of Winform/WPF dialogs, a library that I've not seen before. I doesn't look like it's been updated in a couple years, but the source is provided and the license looks very friendly, so have at it!
Here's some snaps of the mentioned sample app.