At heart, Yags is generic system based around a simple mechanic and lots of player options. There is a wealth of supporting documents and it is updated regularly by the driving force of Samuel Penn.
The simple mechanicThe system is both familiar and elegant, roll D20 + (skill * attribute) then compare to a target number. Therefore, its weighted more towards the skills/attributes rather than the random roll (especially at higher skill levels). Attributes are at the core of a character's description are classed between 0 (no ability) through to 9+ (divine). They are split between eight primary attributes (which start at values of 2 to 5) and three secondary (that have no limits). For a generic system, the attributes are well chosen and general enough to fit most settings but are not so vague that your character is made from 'Stuff' and 'Thingy'. Skills are learnt through practise and training, you can gain more as the games goes along. Nothing ground breaking but the familiarity is a welcome comfort blanket to many. There are lots of examples of target numbers and the concept of degree of success (the more you're over the target, the better you succeed).
The Core books
The books are produced in HTML and PDF. They are identical as far as I can see but I prefer the layout of printed PDFs, so I will only review those. The Core PDFs are attractively produced two columns in three separate books (Core, Character Generation, Combat) . I'd rather all of the things you needed for Yags were in a single PDF: Download it. Print it. Run it. There are also separate files for Modern Character creation and the Character sheets (one for fantasy-esque, one for sci-fi).
The Core books are colour coded (see Core rules right). It's not really needed but I like it. In the Core rules, Samuel describes his motivation for taking certain decisions in the game. Interesting as a writer and useful as a GM but perhaps best served either on the website or in a section at the back of the book.
The Character book is well put together, there are lots of well described advantages, disadvantages and skills. Plenty here to build just about any character from any setting. Perhaps the skills could be cut back to allow sub-settings to define their own. If there is to be any crunch, it is in the sheer number of skills available.
The combat system has a fair amount to it and I like the various options. I would like to see the more advanced rules obviously separated from the standard rules for the new GM but that's a minor complaint. It uses the same resolution mechanic as given above with different modifiers. There are examples for the more thorny areas and it would be nice to see some examples of a section of combat as a whole.
There are pages that have only a little bit of information on it and then nothing else. It seems that each section starts on a new page, which isn't really necessary. Would be nice to remove the white space to save on home printing. I'm utterly guilty of this with Icar. Samuel has included the Gaming license at the back of the Core rules. As it stretches over 2 pages of legalese, it might be worth moving it as a link on the website (which there is) or as a footer in the PDF.
The books are sadly bereft of images but I don't see that as a big problem. It might put some off but the game stands well without it.
Supporting materialI'd make a cup of tea and take a seat before you plough into all of the additional material because there is enough to fill a library. 17 books (in PDF and HTML) covering a multitude of settings (modern, low mythic fantasy, sci fi), equipment and bestiaries and so on. Group by setting type (fantasy, modern etc). There's enough content in each for a review by themselves. Oddly, there is no generic fantasy - something I rather like. The default setting for any free RPG tends to be a stock fantasy, so it's good that Samuel has steered clear of that. He notes that the system was created alongside Habisfern, the mythic low-fantasy setting. It has plenty of flavour to it and has the feel of something that ran as a campaign for many years. As a good example of the breadth of support, Samuel has also included a guide to making your own Yags books, which is both inspired and indicative of the game itself.
The websiteThe website can be a bit sporadic in its navigation but the dates of updates and news shows that Samuel is still working on the game (see right taken at time of writing). I'd like to see download pages consolidated and an easy 'get it here' link from every page. Minor niggles that might put some off but didn't make me want to tear off my own head. I can see that there is a redesign in process with white-and-blue pages being newer and green-and-white being older. Consolidation will soon wrap them all up.
Final thoughtsIn conclusion, it's a well thought out system with a simple mechanic that is used throughout. The PDFs are well written, terms explained and rules laid out logically. The only support that is a bit tricky to fathom is whether it would run a good Supers campaign. I'd imagine so with the levels set the way they are but I've never run Supers, so I am somewhat ignorant. It's not really crunchy. There are a lot of options but they follow a similar mechanic, so your brain is unlikely to melt. I'd call the mechanics simple yet deep.
Yags is polished, free and Samuel should be applauded for sharing!