One of the challenges with a SharePoint environment is managing the security. Who can see what? Can a person see what they're supposed to see?
SharePoint's out-of-the-box interface is limited in this area. Sure, you can tell which SharePoint groups or Active Directory groups have access to a resouce, but there's no easy way to find out who belongs to those groups from within SharePoint.
SharePoint Sushi (love the aconym - it stands for SharePoint Utility with a Smart Helpful Interface) is a great free tool which lets you view your SharePoint permissions from the user perspective.
http://sushi.codeplex.com/ for the download and docs.