I am a fan of open source tools and i really have great admiration for people developing open source tools.I always believed one of the success factors for java is also the support provided by the open source community.
The other day i was reading a very active debate going on at www.theserverside.net on "Does the .NET community need open source?".I firmly believe a very vibrant open source community for .NET will be very good for both Microsoft as well as the developers.I truly believe it's a win win situation.And i really don't believe the arguments against open source and i don't see how an active community creating some great tools and releasing it under open source for the rest of the developer community can be bad for .NET.
Currently most of the java open source projects have their .NET counterparts developed or under development. For example:
junit - nunit (an unit testing framework)
hibernate - nhibernate (object relational mapping tool)
ant - nant (a build tool)
and a whole lot more...
So if you want to check out the open source tools available on .NET try the following links