There have been a number of times where I wished I could hide a module from a user if they are logged in.   Now, Joomla makes it easy to hide a module from non-logged in user, but it doesn't provide a way to hide a module from a logged-in user.

For example, let's say you want to make the "Log-in module" disappear if the user is logged in.

A user on the Joomla forum pointed me to a fairly simple modification (or hack) that can be done that makes this possible.

With a few minutes of cutting and pasting from this forum posting into a few Joomla core files, you will have added this feature to your site.

Here are the instructions for how do to this:  http://forum.joomla.org/index.php/topic,114630.msg795120.html#msg795120