YAGS is a generic rule system, but cannot live without actual campaign settings to use it in. Originally, YAGS was designed alongside the Habisfern setting - a mythical dark ages environment designed for realism and low magic. Since then, it has also been used for Babylon 5, HarnWorld and Traveller.
Two of the settings are still in active use, and have been updated for the latest version of the rules. The others are no longer played, and used a much older version of the rules.
Rules for Babylon 5 were the first attempt at a science fiction version of the rules. They are a very early set of the rules, and have many differences from those currently in use. The most notable difference is that attributes averaged at 12 (instead of 3), and skills were represented as a percentage (in multiples of 25%) which multiplied the attribute. This gives very similar results to the current rules, but with more fine grained attributes at the expense of more complex mathematics.
The most interesting part of the Babylon 5 rules was that they introduced support for Psionics and early rules for firearms.
A fantasy conversion of YAGS for HarnWorld. This was the first complete magic system for YAGS, and also included a religion system for priests.
The rules for Harn World are not currently available as published documents, but are available as XML source files in subversion.
Call of Cthulhu
A brief Call of Cthulhu game was run with a simplified version of the YAGS rules. This used a simplified attribute system that became the standard system used in YAGS today. In theory, it is possible to simplify YAGS quite quickly - simply removing the techniques would remove a lot of the rules and complexity of character generation.