(This page is a work in progress)
FusionForge is mainly developed on Linux systems, but it should run on other Unix variants as well. The minimal required components are Apache 2, PHP 5 and PostgreSQL 8.x. Other components may be needed depending on the extra features you need: Mailman for mailing-lists, CVS or SVN for version control, and so on.
There are *.deb packages available for Debian and derivatives such as Ubuntu. The official Debian repositories contain packages for Debian unstable (in the experimental section for now). Users of Ubuntu or other versions of Debian will find instructions on how to get the packages on http://fusionforge.fusionforge.org/ (in the Debian or Ubuntu tabs).
- .rpm packages are being worked on, and they will be advertised as soon as they're considered ready.
If you have a server you want to fully dedicate to FusionForge, then the installation CD could be for you: just boot the server on it, and it'll perform an automated installation of Debian + FusionForge. Grab the image at http://fusionforge.fusionforge.org/
Also easy to use is the VMware image, which you'll also find at http://fusionforge.fusionforge.org/
Finally, there's the non-packaged way of installing FusionForge: grab the tarball, then run the install.sh script.