I've read Kingmaker, and I've heard of Birthright. I'm wondering what other rpgs are out their in that vein as those are the two I keep stumbling upon.
I'm interested more in the type of mechanics and activities each system focuses on. What are the rewards for the players to spend time building a kingdom? Are they cooperatively focused? How is the pacing? What makes the rpg stand out?
edit: reworded question and details, it was too broad.
There are two ways to approach a "kingdom building" campaign. One is from the political side, and the other is from the build-and-supply side. The former focuses on social maneuvering and only touches lightly on logistics. The latter focuses on logistics and only touches lightly on social maneuvering.
If you're looking to run a dramatic political campaign, my favorite reference is Dynasties & Demagogues (for 3.5E) by Chris Aylott, published by Atlas Games. It gives solid advice for running a political campaign without it getting bogged down in boring stuff. It adds politically-oriented spells, personality feats and other feats, leader-based prestige classes, and a political maneuvering system that is a lot like social combat.
If you're looking to run a campaign where characters focus on building strongholds and dealing with logistics, my favorite reference is Stronghold Builder's Guidebook (for 3.5E) by Matt Forbeck and David Noonan, published by Wizards of the Coast. It focuses on the stronghold itself: building it, defending it, mapping it, and so on. That may be more limited than you want for a "kingdom building" game.
More like Birthright is Strongholds & Dynasties by Paul Tucker, published by Mongoose Publishing. It provides a system for governance, economy tables, rules for building strongholds, a mass combat system, and so on. It's pretty complete and solves many of the same problems as Birthright, but in a different way.
In response to the question of "What defines good rules" as far as the Song of Ice and Fire RPG...
(And I assume that the particulars of the setting are unimportant for the sake of the question, as while it is important to how things function, it's outside the scope of the basic question.)
In the game, the characters are assumed to be part of a noble house governing specific lands within the setting. Characters can be made to interact heavily with these rules (as in, they are concerned specifically with maintaining the reputation and fortunes of the house) or they can roughly ignore them, depending on the focus of the game. The Kingdom Building aspects of the game are as important or peripheral as individual tastes require.
The Houses are defined according to their individual Defense, Influence, Lands, Law, Population, Power and Wealth. The individual scores are generated according to where in the setting the Noble House is located, how long the House has been in existence, what sort of origin granted it nobility, and what sort of events have defined its history. (These are mostly generated randomly, although they can be picked with GM approval.) These scores are maintained through periodic rolls and PC interaction as necessary.
From these scores, certain qualities can be purchased, such as the specific castle features and land features. (Does the Noble House actually have a castle, or just a simple hall for feasts and gatherings? Where is it located? On a river for trade? Near some old ruins? How many smaller Houses depend on this one for defense, and what is the make-up of the militia? And so on...)
As the game progresses, the house fortunes may rise and fall, and depending on the direction of the game, they can upgrade certain features, which can take a period of time. As such, this can define how events impact it. Building a castle keep for defense can take as long as 17 years, where building a marketplace and attracting traders to your town might only take between a couple of weeks and six months.
And being that Song of Ice and Fire puts a great deal of emphasis on social interaction, I feel that it reinforces the "kingdom building" aspects of the game by requiring that interactions depend more on the characters' abilities as diplomats slightly more than how well they can swing a sword.
Which, considering the source material, only makes sense.
Reign, as mentioned by others, uses the "One Roll Engine"... you'll need a bunch of D10's, and the interpretation of rolls is odd on it. More flexible than birthright.
John Wick's Houses of the Blooded also covers building large domains within the setting, and could easily do it much like Birthright, but with a MUCH more narrative scale. Blood and Honor is the same engine, for Japanese play, simplified slightly.
D&D Cyclopedia has some rules on doing so in a D&D context... they're not great, but also were done pre-birthright, and are simpler in execution.
Pendragon is below kingdom scale, but again, does so. Focus is much more realistic than the above; system is a 1d20 variant of BRP (which is 1d100 driven). Expect to take years building a decent castle... but given the time compression, you'll have it if you can get the cash. Problem is that the landholding rules are not intended for starting PC's, but can be done in some cases. Also, supplements are required to do so past the single manor level.
D&D 3.0 had a strongholds supplement that could do much of it, but it isn't as clear as the above.
And going back to the 1981 monster game, Chivalry and Sorcery allows simulating fantasy kingdoms on any level you opt for... later editions have more codified rules.
Mongoose Runequest 2 recently released its Empires supplement. It's a percentile system derived from BRP that draws up a list for each political entity very similar to a character profile. Characters can control factions, guilds, and a whole range of political systems from villages to empires (you can also play polities that are nested within another, like a province of an empire). They can gain renown within these systems, and go on missions that will affect their growth.
The time per turn is about one year, in which a state can attempt a fairly large number of actions limited mainly by its revenues. Wars are composed of "campaigns", which tend to happen on a shorter timescale. There's an emphasis on building up your faction / state to the greatest extent possible, but there are also rules for collapse and revolution that get worse the larger it gets.
The emphasis is on Glorantha 2nd age but the rules are easily adapted to other fantasy systems. In the back of the book, they stat up all the provinces of the Roman Empire using the system, which is pretty cool.
Greg Stolze's Reign has a lot of emphasis on this, and may be the single best choice.
Green Ronin's Song of Ice and Fire RPG has extensive and very good rules on building and running a noble house. Slightly more specific, but definitely in the same family.
Lastly, Empires was a d20 supplement that basically brought birthright rules into 3e.
Brief Elaboration: Reign uses the One Roll Engine (ORE) system which is a set-matching mechanic with no math beyond the values on the dice. This is important because it has allowed for a system which scales up easily, allowing the same mechanic to be used for handling armies, nations and other elements of scale. The game was built on the foundation of this, an even the subtitle (a game of lords and leaders) points to that emphasis. In short, it does exactly this, with a well tuned system (ORE has been pretty well put through the paces), and comes from the mind of Greg Stolze, which is a selling point and a half right there.
The SIFRP system is more what you would suspect a complex subsystem that makes the act of building a great house something akin to building a character. The system is complex enough to allow a lot of variety in size, type and nature of houses, as is apt given the source material i represents. In addition to making for distinct, well detailed houses, the system of creation also seems well designed to engage all players in a group.
I just picked up a PDF of a game called Adventurer Conqueror King System (ACKS, pronounced, "axe"). The primary draw for me for that game was the attention paid to economics.
I haven't even gotten to read it yet, but it seems to be an osr game with an emphasis on the fact that in the original D&D, characters were supposed to grow into positions of power and influence. The name of the game is that progression: Adventurer, Conqueror, King. So the game is supposed to support gangs, armies, strongholds, etc..
The blogs at the official site give some details about the money system that were instrumental in making this a must buy for me. How much is a realm worth? How much does a peasant earn? That sort of thing.
I'm suprised nobody's mentioned Exalted. You'd almost have to specifically try and avoid kingdom-building to not get at least some of it in a Solars game; I'm not personally familiar with the large-scale mechanics, but I know they exist, and they're pretty darn important to an RPG in which all players are playing the reincarnations of the overthrown rulers of all Creation.
Jeremy Keller is working on a lordship supplement to his game Chronica Feudalis, called Noblesse Oblige. There's not much information out about it yet, but I'll update this answer as more details emerge and the supplement is eventually released.
The key thing that got my attention is that it seems to have the right level of granularity for my taste: there is a list of specific structures you can build in your domain, which can be leveraged for advantage or targeted for defense/destruction within the battle mechanics. Prep also appears to generate a relationship map. So the play seems like it will be interpersonal drama at one level, with Civilisation-like granularity at the domain-building and battle level.
