Is there a standard message box in WPF, like WinForms\' System.Windows.Forms.MessageBox.Show(), or should I use the WinForms message box?
As the others say, there is a MessageBox in the WPF namespace (System.Windows).
The problem is that it is the same old messagebox with OK, Cancel, etc. Windows Vista and Windows 7 have moved on to use Task Dialogs instead.
Unfortunately there is no easy standard interface for task dialogs. I use an implementation from CodeProject KB.