We can not deny Java having a flourishing open source community. But .Net has it's counterparts to.
Sometimes they are pretty easy to find. Just add an "N" to the front of the java projectname, or add .NET to the back:
- Hibernate <-> NHibernate
- FOP <-> NFOP
- Cruisecontrol <-> Cruisecontrol.NET
- Ant <-> NAnt
- Spring <-> Spring.NET