YAGS is another game system, with a moderately crunchy set of rules designed to give a gritty cinematic style of play suitable for modern day, fantasy or science fiction genres. YAGS has been around for over twenty years, and is available for free under the GNU General Public License. This means that you are free to download, modify and republish the rules however you want, as long as all versions are released under the GPL. The source for YAGS can be downloaded from GitHub.
YAGS is not designed to be a lightweight set of rules, but has been streamlined to try and keep it no more complex than it needs to be without sacrificing rules flavour. So though it's a lot more complicated than something like Fudge, it's not as complicated as GURPS.
YAGS has been used to run a number of different games in a variety of settings and genres:
YAGS aims to be simple, but also aims to capture certain flavours of the real world using rules where necessary. It is not as simple as games which aim to be simple, and represents a lot of things with hard numbers.
YAGS can be deadly, and even the greatest hero can be felled by a well aimed bullet or unseen sword thrust. However, an experienced character which is played intelligently can cause carnage on a large scale just as Beatrix Kiddo, Azumi or Conan can.
The core system makes use of a simple task resolution mechanic. You take an attribute, multiply it by a skill and add a 1d20 roll. This is then compared against a target number to see if you succeed. The more you succeed by, the better the result.
A typical professional will have an attribute of 3 and a skill of 4, meaning that they roll 12 + 1d20. An exceptional character may have an attribute of 6 or 7, and someone who has mastered a skill may have a skill of around 12-14. This means that there is a huge gap between standard characters and exceptional heroes.
The most current version of YAGS is version 0.7. It was last used to run an SF campaign set in GDW's Traveller. Rules for modern combat, vehicles and spacecraft are the areas which are the focus for this setting.