Free or Open Source Diagramming Component for WinForms

Did you try the CodeProject article series by Sukram:

WPF Diagram Designer

Too late for the questioner but perhaps interesting for researchers:

NShape

NShape is an Open Source diagram designing framework for .NET WinForms. Software developers use NShape to integrate diagramming capabilities into their applications. Using NShape, applications let users view, annotate, modify and create diagrams like flow charts, wiring schemes or project charts. NShape is open source and has a dual license which allows it to be employed for free in open source projects and for a license fee in commercial projects.