Here's a Link:
http://web.wt.net/~chess/gnuchess.html
This one is for windows users.
You can pretty much get a version for every computer since they all do use the same library stuff, underneath. It basically boils down to interface, for why the different versions.
http://www.tim-mann.org/xboard.html
has more information on the GNU chess stuff.
You can search for GNU frontends for ages. Some people have really snazzy and pretty interfaces for the engine. I remember even the older Amiga ones looking quite nice with their nicely shaded high color look.
GNU is one of the oldest and longest running Chess programs. If not the oldest. It's been updated for quite a few years. I like to call it the "NETHACK" of Chess games. Over the years, the red and black books have constantly updated typically keeping GNU as one of the forefronts in powerful chess AI. And like, some others, it has the ability for the user to set how much thinking is used for the CPU to think out the moves. How far ahead does the Computer plot its strategies, etc. So if you knew about how many moves ahead an opponent would use, you can sort of use this as an idea of what you may go up against. (Within reason of course, the specific personality of your real live opponent could differ greatly, pulling some odd 6th match budapest when the computer wouldn't, etc.).
Of course, the majority of the programming in GNU is all about making the CPU as smart as it can be, while adding new strategies to the red and black book the CPU uses and is aware of. Various ongoing releases adding some new style, more recently discovered, in the world of chess. Many times, and in many cases, GNU has been used as a kind of benchmark for how far CPU chess algorithms have come.