Currently there are quite a few DI/IoC-frameworks for .NET out there (http://www.hanselman.com/blog/ListOfNETDependencyInjectionContainersIOC.aspx). I find it quite hard to choose. Therefore I wanted to measure the public opinion and see which framework is the most popular - so please post your favorite framework here and let the people vote...
Use Windsor or Unity, and decide based on a POC
Unity is not so bloated up so far, you could go with that based on your project type - especially if you are using unity dependent frameworks like CompositeWPF
See another thread on SO about the same, http://stackoverflow.com/questions/411660/enterprise-library-unity-vs-other-ioc-containers/552237
I'm currently using Castle.Windsor. Mainly because of the integration it has with the Rhino Tools library.
I've been watching Kzu (creator of Moq)'s screencast series as he develops Funq - a DI framework that makes extensive use of lambdas the same way Moq does. No release yet but I'm looking forward to trying it out.
StructureMap
I love the fact that I can configure it all using the fluent interface (aka Configuration DSL). Typesafe configuration is a must when I do refactoring.
Unity It's simple and unintrusive, yet type-safe. And it is created by Microsoft Pattern & Pratices group.