Is there any way to automate windows forms testing?

Check out http://www.codeplex.com/white and http://nunitforms.sourceforge.net/. We've used the White project with success.

Same Answer to a previous question

Edit

The White project has moved, and is now located on GitHub as part of TestStack.

WinAppDriver is a Selenium-like UI test automation service for testing Windows applications including Windows Forms applications. It can be used with Appium, a test automation framework.

As a new alternative, I can give you FlaUI (https://github.com/Roemer/FlaUI). It is basically a complete re-write of white with more features and a clean code-base.

AutomatedQA's TestComplete is a good testing application to automate GUI testing. It supports more than just Windows Forms also, so you can reuse it for other applications. It is not open source and this is the best that I have found. I haven't seen an open source equivalent to WatiN. It does have a free trial, for you decide if you like it or not. The main reason I went with it, is that it really is cost effective, compared to other testing applications.