I recently found this site and seems to be all about the Open Source applications around for Windows.
The site neatly categorises the applications. Also if you ahve questions on the Why and What of OpenSource this might just come to help. I havent been through this site completely yet. here is a snippet from the contents listing in the page.
>"Free" vs "Open Source" Software
>Why I went Open Source
>How Open Source Software Evolves
>Does Open Source Offer Too Many Choices?
>How do Open Source Apps Measure Up?
>Running Windows Apps in Linux
A neat and good place to visit it seems. Though mind you the list of applications is not very exhaustive here. so if its just applications then you might just look here :-)