Sitecore is a .NET CMS system developed in Denmark by Sitecore A/S. We are starting to use the product more and more at work.
Today I attended the second Sitecore development course in Copenhagen, "SCD2". So now I am a certified Sitecore developer ;-)
Sitecore is a rather good CMS system (in my humble opinion). Some of the forces is that is easy to extend, and extensions can be created in well known technologies such as .NET controls and XSLT transformation (amongst others), and one can choose whichever is best for the task at hand. My impression is that it has a good and well-structured API, and that it is very stable. I am looking forward to do some real development work using the techniques I learned today.