string ClassName = "SomeClassName";
object o = MagicallyCreateInstance("SomeClassName");

private static object MagicallyCreateInstance(string className)
{
    var assembly = Assembly.GetExecutingAssembly();

    var type = assembly.GetTypes()
        .First(t => t.Name == className);

    return Activator.CreateInstance(type);
}

private static IEnumerable<Type> GetDerivedTypesFor(Type baseType)
{
    var assembly = Assembly.GetExecutingAssembly();

    return assembly.GetTypes()
        .Where(baseType.IsAssignableFrom)
        .Where(t => baseType != t);
}

Activator.CreateInstance(Type.GetType("SomeNamespace.SomeClassName"));

Activator.CreateInstance(null, "SomeNamespace.SomeClassName").Unwrap();