StructureMap is very easy to use, but it makes creating loosely coupled OO systems a breeze (ok, that’s exaggerating.  it still requires engineering).  I use it in every project I work on, and I like it better than Spring.Net.  Spring needs Xml configuration for every mapping between an interface and a class.  This causes the Xml file to continue to grow and grow and grow.  StructureMap uses Attributes to tag the interface and class definition.  At runtime, it hooks them up.  This type of metadata is much easier to manage.

Jeremy Miller recently released version 1.0 of StructureMap.  I was please he included a feature I explicitly asked for.  If you need a port to .Net 2.0, let me know.  I’ve done it.