Codex Martialis is an OGL supplement for Dungeons and Dragons. The purpose of CM is to revamp melee combat to make it both more interesting and more realistic. While I haven't gotten to test it first hand (my group is still enamoured with the new hotness that is 4th edition and is currently unwilling to go back to 3.5 even with new supplements) CM appears to have succeeded on both counts.
The first change is the Martial Pool. Players get a number of d20 dice to spend on each round of combat. The dice can be split up into individual attacks or merged into one powerful attack. They can be stowed away for defensive maneuvers or spent later as a counter attack. These rules are not unlike the combat pool systems in World of Darkness and Shadowrun. MP rules are given in one page, with a few additional paragraphs through the book, and already improved d20 combat. Unless your character is built for certain maneuvers or put in specific siuations, a D&D martial character has few options which usually boil down to stepping closer for a full attack or charging someone, all while trying to gain a flank. That's it. And it's terribly boring. The MP pool gives melee characters more choices to make each turn, which alone should make melee combat more interesting.
Combat now takes place across a set of ranges - onset, melee, and grapple. Anyone who has fenced before knows that range plays a bigger part in a fight than simply letting someone hit you from two squares away. Weapons have reach bonuses, which add to your attacks when you have a reach advantage and defensive bonuses, which apply until an enemy closes the distance. This means that a rapier will play differently than a longsword, besides just doing different damage.
Combat ranges are not perfect though. Since they break out of the distance based combat, I have to wonder if you would still use a grid to represent battles. It seems like it was designed for fencing, but fencing takes place between two combatants. Could three people grapple? Or would one of them remain at melee range. I should point out that I'm not saying that this part of the game doesn't work, just that I'd like a little more guidance about how to implement it.
The book also includes a great number of martial feats, which are given to the players in addition to their normal feats. These feats represent techniques and maneuvers used in martial combat. They focus on giving characters more things to do, instead of improving the things characters already can do. As I mentioned in the MP paragraph, I wholeheartedly support giving melee characters more options.
Finally, what would a Codex Martialis be without weapons and armor. Almost the final third of the book (not counting license or index pages) is tables for equipment properties. Weapons have the reach and defense bonuses previously mentioned, but also gain a speed property for counter attacks. The book includes more swords than I can pronounce, and the swords are only a fraction of available weapons. I have a sneaking suspicion that the weapons are statted for realism instead of for game balance (ie, the only reason to use a sickle over a sax sword is if the latter is our of your price range) but there are enough weapons with enough different strategies that I can't see any one weapon being optimal for all situations. As complicated as this may seem, the weapons section reminded me of some of the elegance of this system. Ranged weapon prep time is done measured in dice from a character's Martial Pool. This means that reloading costs a fraction of a turn - a welcome change since players balk at anything that uses up their combat actions. It also means that characters get better at reloading. A bow may be the best choice at low level, but as your MP grows you can afford to spend more of it on reloading.
If you're looking to run a more realistic D&D game, I can highly recommend Codex Martialis. It gives players the options needed to be unique and interesting fighters with varying techniques and styles. While I'm not a huge stickler for realism I can appreciate the fact that this material is historically accurate. And even more important than that, the realism adds to the game where other sources of realism would obstruct it.
And if that's not enough, you should also check out Codex Martialis Weapons of the Ancient World. This book expands on the first by explaining the function of many of the weapons listed. It even has pictures. This won't add new rules to your game in the same way the original book did, but it will make the weapons come to life as something more than a bundle of stats. Even if you don't use the Codex's ruleset or ever play another RPG, this document is a fantastic resource and a good read when it comes to the historical use of weapons from all over the world. I just hope there's a sequel concerning armor.
[4 of 5 Stars!]