[XsltExtension(Namespace="myAlias")]
    public class SomeTest
    {
        public static string HelloWorld(string who)
        {
            return "Hello " + who;
        }
    }

http://umbraco.org

Currently the XsltExtension attribute will only work when the class is placed in the app_code folder, in umbraco Juno, we'll also look at types in assemblies, so no more need to update the /config/xsltExtensions.config but simply mark class with the attribute

Umbraco Juno, XsltExtension attribute (now also searches in assemblies, not only app_code)

C#