There is codedui which you can use with Visual Studio/Visual Studio Test Edition. Although it's not open source, it works with your existing .Net dev environment.
In terms of frameworks or tools you that allow you to extend it's core technologies, I'm drawing a blank thinking of any. .Net framework if pretty proprietary (although popular) in itself, it's very hard to find open source UI frameworks that allow you to automate.
I know it's not open source, but if you are coding in .Net for application delivery wouldn't you have Studio available? That has its own automation.
While I have been working for Businesses that are Vendor partners with HP, IBM and Microsoft, my opinions and advice is my own.
The solutions provided are either sourced from my own scripting libraries or from a quick Google Search.