OpenSourceAutomation.com is a Windows .NET open source project. They have only just released their first beta, but it may hold promise for Windows users looking for an open source system. I would not even test it as an end user until they get a few maint releases out, but if you are looking for a long term system you can develop against, it might be the way to go.
(Yes, I am biased towards this project as the co-founder of OSA and the creater of the vCrib project that inspired it =) This project is still very young, hence my silence on here about it. I will come back and post announcements as the system is ready for public consumption, but welcome any hard core users or developers to check it out now. Next beta release is Jan 31st, but repository code is updated daily...
Vaughn
www.OpenSourceAutomation.com