FreeSpace Open Installer
The FreeSpace Open Installer is currently the only official online installer for FS2_Open. The Installer was originally written by Turey, and then completely rewritten by Goober5000. The source code is hosted by the SCP.
The program's web page is here.
Instructions on creating an install.txt file are here. These are only for modders who are ready to add their released mod or campaign to the installer's list of available mods to install.
A mirror monitor for the various mods can be found here.
Both links contain the same installer. The program requires Java version 1.5 or higher, and can be run on any operating system that can run a compatible version of Java.
You can access the source code from https://github.com/scp-fs2open/fsoinstaller.