I have been using Spring.Net for implementing Dependency Injection in my .Net projects. Spring is already one of the most known providers of DI in the Java world and a port to .Net actually eased the lives of people like me who are willing to develop unit-testable software.
However, with all the happiness enjoyed through Spring.Net, I guess its time to take a look into Unity. I have only started using this and I found it interesting in the sense that its more of .Net flavored with a few attributes and fluent interfaces.
I hope to post a few concrete examples on Unity implementation soon. However, you may wish to take a look at http://www.codeplex.com/unity in the meanwhile and let me know your comments on this new application block by Microsoft Patterns and Practices group.