I thought I'd start a series on the game systems I have experience with, so we'll start with my college friend, Mark Christiansen's Cold Iron. Cold Iron has never been published, in fact, Mark actively discouraged attempts to provide copies of the rules that others had crafter out of a legitimate desire to avoid people telling him how HIS game worked based on some rules set that incorporated misunderstandings and house rules. Eventually he softened his stance (eventually even using the same computer document for the spells that I have been working from). Still, the game has never been published by Mark. For my own campaign needs, I have published my rules on my Cold Iron page
I was first exposed to Cold Iron when another friend, Rob Hendrie, started a new campaign (Mark had already graduated by then). He had an early copy of the spells plus the combat, weapon, and skill charts. He had some handwritten pages with some magic items listed.
Cold Iron as I orignally saw it was basically just a combat system. Sure, the spell list included some random non-combat spells, and I think Rob included a scouting skill. The system very clearly derives from D&D, however, there is definitely a lot of influence from Rune Quest.
Cold Iron is a transitional system between pure character class games (D&D/AD&D of the 70s and 80s) and skill based games. Every character is multi-classed. Every character has a fighter level that determines their hit points and weapon skill. Every character also has at least one magic class. There is a passive magic class that only represents the character's magical defense and general knowledge of magic. Spell casters are either magic users or clerics (or both) with appropriate class levels. Experience in all magic classes is totalled together to determine effective passive magic level. Then there are the combat skills. By Mark's rules, these are actually separate skills from fighting level, but every fighter may either improve one fighting skill with his fighting level, or he may spread XP based on his fighting level amongst two or more combat skills. Every game I ever played in also had an additional class level, humor level. I'm not quite sure why it was called humor level, but it represented one's personality strength (going along with Charisma). I think many games also had a scouting level.
Characters have attributes, with Alertness and Size added to the standard D&D mix of Strength, Dexterity, Constitution, Intelligence, Wisdom, and Charisma (later Willpower was also added). An additional attribute, MP (mana points) was used to power spells and most magic items. All attributes except Size and MP were rolled on 3d6. An additional d6 was rolled and added to the sum as a Potential that the attribute could be improved to. Size was generated as 4d4 - 10 + (STR potential + CON potential) / 2. MP was generated as 6d6. Most games allowed you to roll 4d6 take the best 3 (no re-organizing or trading points though) for the base, and something like 2d6 take the best or 1d6 and 1d4 take the best for potential. Different races had various different ways of rolling things (Dwarfs got 3d6 for MP for example).
At the start, a player could increase one attribute to potential. Every class level above 1st also allowed the character to increase one attribute one point towards potential, sometimes with limitations of what attributes you could increase (for example, humor levels could only be used to increase Charisma). This was called augmenting. Spell casters could also augment MP (though mages also got +2 MP per level, including 1st and clerics and passive magic got you +1 MP per level), MP has no upper potential. Fighters who ran out of things to augment could augment hit points.
Spell casters were not limited in use of weapons and armor. Spells are mostly single target, and usually it's more efficient for the mage to do something that makes it easier for the fighter than directly doing damage. Spell casters get new spells each level, and can memorize a pretty wide selection, using their MP to cast them.
One of the features I find most interesting in the game is the use of the normal distribution for resolution. People have long played with using various numbers of dice to create bell curves, and various schemes for exploding dice (if you roll the highest number, roll again and add) for open ended results. Mark realized that when you roll d100, trying to roll less than or equal to some number, you can visualize that ever so slightly differently. If you visualise the target number as a probability (between 0 and 1) and the dice as generating the first two decimal places of a real number between 0 and 1, you basically get the same thing (and d1000 is just rolling 3 decimal places). What if you roll a d10 for each decimal place in a real number? You could roll as many dice as you wanted. Mark then took this idea and created a normal distribution with a mean of 0, and a standard deviation of 20/3. He rounded each number so that it had two digits beyond a leading string of 0s (for the low end) or 9s (for the high end). Here is an Excel spreasheet
that shows how the chart is computed (thought Mark didn't have such a convenient way to compute it - it's interesting to see how close his chart is to what Excel computes, column E shows the rounded Excel values, column F shows the three values that were different on Mark's chart). This chart sometimes scares players away because most people do not find probability and statistics intuitive, but in play the chart is real easy to use. Roll a pair of d10s, making it clear which one is read first. If the first die is a 0 or 9, roll a 3rd die. If the 2nd die is the same 0 or 9, roll a 4th die. If you roll 3 or 4 0s or 9s, you've done really well, roll additional d10s until you can pinpoint your resulting number as greater than or equal to one of the numbers on the chart, and less than the next one (so if I roll a 9 3, I roll one more die, a 4, look at the chart and read a +10). The way the game tends to work, rolling less than a 5 0 rarely succeeds. I have found that when I am playing regularly I can remember enough of the chart that I hardly need to look at it (I can also remember the chart well enough that I can come pretty close just writing numbers down blind - I also have a wallet card with the chart...).
Another feature I really like is that most of the treasure goes to purchasing potions and charged items. This dramatically reduces the growth curve of magic items. Also, all the magic items are directly implementing spells. Additionally, since they require the user to spend the MP (except for potions), there are no problems with overly cheap magic items, in fact, some magic items end up being impractical. Magic swords use slightly different rules to make them practical.
These days, I have only two fundamental problems with the basic system. First, I have come to dislike random character generation. Fortunately, it's easy to use a point build system (it can even have linear costs and works ok, in fact, I think it's good to have linear costs, geometric costs like used in D&D 3e may encourage too much sameness in characters by discouraging the highest attributes). The second problem is a little bit more of an issue, and that is that it's too optimal to be a spell caster. It doesn't cost enough of your fighting ability (though point buy attributes would actually cure a lot of this, still, everyone would want to be a caster even if they just had the minimum spell casting attributes).
Back in college, I refined the skill system by breaking down the combat skills and giving non-casters 5 combat skills (which amounted to 2.5 skills by the old system). Magic users got 3 combat skills, and clerics usually got 4.
I also added more non-combat stuff. That introduced several problems. First, it was never really clear how to use some of the non-combat skills. I was quickly on my way to realizing the "locks and traps" "thief" of D&D was a diversion to the game. The character existed to deal with the increasing use of locks and traps, but had to be justified by making locks and traps even more important, so you could no longer play without a thief. In my Tekumel campaign, I had one player create an aristocrat that basically was useless in a fight. It wasn't a pretty sight when he tried to use his charisma to get someone else to take care of the problem they had stumbled on. It was less pretty when I tried to push him into combat (at one point, he was standing by watching while two characters struggled with an undead, had he stepped in between the two characters, the undead would have been prevented from parrying one of the two, relying on dodge only).
Cold Iron does tend to have somewhat long fights. It also simply doesn't handle non-combat stuff well at all. But as a gamist system it really works well. Before I got involved with the game, it had already been through 3 or 4 years of hard-core gamist play with several GMs running games. The worst of the "holes" had been patched. Bunk choices were either eliminated, or well recognized ("Don't do that, your character will suck."). The base system doesn't trick you with lots of "character development" that is really bunk choices (except where I mistakenly added such choices).
I am in the process of writing up new character generation and skill rules that will give players some non-combat ability as a free extra. I'm eliminating the Charisma attribute (in any point buy system that is gamist and combat oriented, I've never seen a serious character have anything more than the minimum charisma). I'm playing around with the possibility that instead of chosing a non-combat ability, the player may choose an ability that makes a less optimal combat strategy more effective. With what I've learned over the past few years about what I really enjoy for game play, and how non-combat resolution meshes poorly with a tactical miniatures style combat system, I think I can massage Cold Iron into something that will be enjoyable to play. I have considered adding some of the ideas of D&D 3e's feats to increase character design choice (while being wary of adding bunk choices). A few simple indications of non-combat ability combined with old school "attribute checks" or whatever will give players enough feeling that there is more to the world than just combat. The key will be to use those abilities to enhance the game. Let the scout find a back door, let the thief open it (quietly) so the PCs get the drop on the enemies (and make a very challenging fight easier - but not eliminating the challenge). Let the scout track the wyvern back to his lair to find the treasure.