When building a website with Joomla 1.5, you might need to hide content from registreted users and guests. Hiding content depending on whether a user is a registered member or a guest is a really useful feature to have.
Fortunately, there are a number of different options available in Joomla 1.5. This article from GoodWebPractices reviews: JoomiHide, Ninja Access and RokAccess.