Maze Rats is a very simple RPG system, but it is obviously a labour of love. The author's stated goal is to make it easy to run fantasy games in the Old School Renaissance (OSR) style. The document is 13 pages long, with tiny print and no wasted space. The writing is clear and concise.
The rules are simple and deadly, to encourage cautious and intelligent play. Characters are defined by three ability scores (used for saving throws), an attack bonus, and a simple special ability. The numbers and bonuses are not very large but the game uses a 2d6 system for attacks and saves, so even a +1 bonus makes a difference.
Despite the simplicity of the rules, the game manages to include virtually all the major features of an old-school RPG system: reaction rolls, morale checks, encumbrance, hirelings, thief skills, and so on. The mechanic used for each feature is simple and logical, so the need to refer to the rules while playing should be minimal. Only six-sided dice are used.
Perhaps the most interesting part of the game itself is the spellcasting system. Rather than choosing spells to memorize, magic users roll on tables when they prepare their spells in the morning. The tables combine keywords to create open-ended spells such as "bewildering dream" or "serpent throne".
The bulk of the Maze Rats document is taken up by random tables for use by the GM. Like the rules, the tables are concise but full of interesting and whimsical possibilities. For example, the random dungeon tables might give you a dungeon in a city sewer, but is just as likely to give you a dungeon in a beaver dam or a giant book. The city might be run by a thieves guild, or it might be run by up-and-coming art collectors. The tables are open-ended enough that they could easily be used with many other RPG systems.
The game follows the philosophy that every monster should be unique and mysterious. Thus there are random tables for monster generation, but there is no bestiary or standard monster list. That said, the rules are simple enough that statting out classic or invented monsters should not be a great hardship.
The book is rounded out with some advice to the GM. The advice is short and to the point -- mainly a compilation of the best ideas you might find browsing OSR blogs.
Overall, Maze Rats is an inspiring work, and one of the finer systems to come out of the OSR movement. I would love to run a game with this system, either as a one-shot or an extended campaign. It would also be a great system to use for introducing tabletop roleplaying to children; the author is a schoolteacher and has been playtesting the game with fifth graders at his school.