Just could not help mentioning about this blog I found today. This is a very exhaustive and well compiled list of open source applications together. Michael has done a great job putting across those reviews of some very useful ( and open source! ;) ) applications. I have decided to permanently link him from my page too.
Visit "Open Source Review" to know more. I am not very sure if he has segregated them on the basis of platforms but am sure you will find that most open source applications are very easy to build on windows too (most of the time ;) )
This is what Michael has to say:
There's a huge wealth of great, FREE(!), open source software out there. There's also a lot of unreliable, incomplete, free open source software out there. I've created this in attempt to set the two apart and spread the word about the great undiscovered software there is out there.