Can you say, "Why isn't this the core functionality of the Joomla! Module Manager"? With AMM, you can expose or limit the display of a module by...well...just about anything you can think of! Even if you only want a module to display on your birthday, AMM can make it happen. We consider this a core extension for any 3by400 site build, and are always impressed with all things NoNumber. NoNumber offers a free version and a pro version for minimal cost.