InstallJammer is a multiplatform GUI installer designed to be completely cross-platform and function on Windows and most all versions of UNIX.
InstallJammer features a very powerful install builder with support for multiple themes and a high level of configurability for installers. Installers are built as single executable files for easy distribution over the web and handle installing everything you need for your application in a simple, cross-platform way.
Active development of InstallJammer has been discontinued. The source is available for all versions and will always remain so, but your efforts might be better spent on an installer that is active in its development.
Some of these links may be of interest:
If you're looking for support, you can try the community forums. Many older topics were archived in a readonly forum when the forums were moved to SourceForge, so you can try looking there for help first, or you can sign up and participate in the current forums.