Open source .NET libraries you should be already using

As we know, there are many good, enterprise level, open source libraries, frameworks and projects implemented in Java. Most of them are being ported into different platforms like .NET and PHP, so developers are migrating to the these platforms with their familiar tools. For example, i have been using Ant, JUnit, Log4J, iText and many mores when i was developing in Java, but now i am still using Log4Net and Nunit in my current