BitRock InstallBuilder Professional builds installations for Windows, Linux, and Mac OS X. It can also build a multi-os installer for a CD installation. The package comes with a single demo project and documentation via a series of HTML files. The product can be run from Windows, Linux, or the Mac. This is perfect for someone like me who develops on a variety of different OSes. BTW, there's an Enterprise version that adds support for HP-UX, AIX, Solaris, and FreeBSD; and individual packages for just creating Mac, Windows, or Linux installations. If you are working on an open source project they offer free licenses to open source prjects.
The core GUI is fairly easy to use with a number of options. Simple installation packages (those that simply need files copied, displays a license and/or readme, and runs some kind of post-install script) can be built using the GUI alone. The actual project file is an XML file that can be customized with additional interactions; there are a number of additional features you can add simply by editing the XML including creating your own screens, changing file permissions, adding environment variables, adding directories to the path, backup files, and more (BTW, there are even OS-specific actions like manipulating the registry). The XML format serves the purpose that scripting languages serve in other installation building packages.
All in all it appears to be a fairly easy to use package. It might not fit every situation, but it appears to handle the majority of situations. Download an eval and check it out.