XML-documentation for a namespace

NDoc supports this by recognising a special NamespaceDoc class located in each namespace, and using the documentation from that. I haven’t tried it, but Sandcastle appears to support the same trick.

Edit:
For example:

namespace Some.Namespace
{
    /// <summary>
    /// This namespace contains stuff
    /// </summary>
    public static class NamespaceDoc
    {
    }
}