So you have a group who just saved the kingdom from yet another threat. As a reward, the king offers them yet another chest of gold. But the group has gotten tired of getting just small fraction of what the kingdom actually has and decides that one of them should marry the king's only child and just take the whole thing.

Soon enough, the king dies and the group that was used to travel through dungeons and slay great monsters finds itself on the throne. They now have access to vast riches and are in control of a massive army. They never have to do anything again, they can just let the army handle it. The game has become very political.

Worse yet, D&D doesn't actually have a simple system to use, or at least none that I know of, which is why this system was created.

First, lets have a look at some terms that will be used here:
Political aspect - a government is divided into several aspects for this system, such as Military, Religion, and Economics.
Political roll - a roll is made for each Political aspect, with possible bonuses and penalties based on the government's decisions.
Public - the inhabitants of the kingdom. This does not include the military forces or members of the government.
Public roll - a roll made to see how well the Public likes you.
Government - monarchy, theocracy, any kind of rulership. This should include all the players of the group.
Mass Combat - unfinished system that allows players to control a military force in combat or war.

Now, lets look at the kingdom's treasury. The biggest problem about having players rule a kingdom is the treasury. The easiest way is to assume that the treasury is endless, but that gives the players access to everything they want. A bit more complicated but so much worth it is to calculate the approximate worth of the treasury. Although a damn lot, the players now have access to limited funds.

Calculating the Treasury
The are few ways of doing this. First is simply to pick a number. Best is to think of the treasury in thousand platinum pieces. A better way is to determine the average expense of each Political aspect you wich to use (see below) per month. This should not include special expenses; this is only the average. Now roll a die, multiply the result with 1000 and add it to the estimated average expenses which you add together. Now multiply the complete outcome with a number you feel comfortable with. 100 or even 1000 should not be out of the question, although smaller kingdoms should be multiplied with smaller numbers.
A kingdom I used to test this used 1000 to multiply with. The outcome was over 100,000,000 PP. It shocked my players quite a bit.
The formula should look like this: (X + YdZ) x T
X = the combined average of every Political aspect to be used.
Y = number of dice to be used.
Z = number of sides on the dice to be used.
T = comfortable number to multiply with.

Political Aspects
Each kingdom should have few political aspects. Good way to determine how many aspects you'd like to use is to have it twice as many as your players. What those aspects are should be up to the DM, but Economics is a must. As examples I'm using 8 aspects in my game: Economics, Agriculture, Education, Justice, Medical, Military, Religion, and Transportation. In my game, Educations covers military schools, clerical training and wizard training. For others, military schools could be covered by Military, clerical training could be in Religion, and wizard training could be in Magic (which is not on my list).
Economics should be in every game where this system is used, because it controls the increase or decrease of the kingdom's treasury.

Political Rolls
One roll is made for each Political aspect every turn. A Political turn is one month, although increasing it to one year should not pose a problem. Each roll's effect becomes apparent in the next turn.
The Political roll is made with Intelligence and the character's level. If you want to make it more complicated, you could have certain Classes have better bonus depending on the Political aspect. Consult the Base Attack bonus table on page 22 in Player's Guide. A Fighter could have a Good Bonus for Military rolls, while Cleric has only Average Bonus for Military rolls but Good Bonus for Religion rolls. The idea is that different Classes have different experiences in different fields, but should always have at least Poor Bonus for any roll.
The base DC for a political roll is 20. Political aspects such as Economics could easily change, while other only change under dire circumstances (see below). Each player should roll for at least one Political aspect, just so that he can still act with the group and not feel disconnected from it.

Certain political decisions should give bonuses and/or penalties to different rolls. A standard decision is to increase or decrease funds for a certain aspect. This results in a bonus for that roll ranging from +1 to +5, depending on how much you want to increase the funds by, but it also results in a penalty equal to the bonus to Economics. So increasing the funds that go into Military by +3 gives a -3 penalty to Economics. Decreasing the funds for the round has the opposite results. Obviously, you can't increase the funds for Economics this way.
Another standard decision is to increase or decrease the taxes. This gives you bonus or penalty to the Economics roll but results in an equal penalty or bonus to the Public roll respectively. So increasing the taxes by +5 gives a -5 penalty to the Public roll. Increasing or decreasing taxes can never give you a higher bonus or penalty on the Economics roll than 5.
Generally speaking, adding a bonus to one Political aspect should give equal penalty to another. Economics is the one who usually suffers, but Agriculture might suffer from increase import. Certain decisions might even give bonus to something completely different, like increasing the Military activity inside cities or towns will not give your a bonus on Military but instead gives bonus to Justice. The DM should be flexible when giving the players bonuses and penalties.

Once all decisions have been made, it is time to roll.
Economics suffers the greatest from bad rolls, but also gains the most for good ones. For every 3 that is rolled above the DC, increase the treasury by 5% (10% for rolling 26, 15% for 29 and so on) to a maximum of 25% when rolling 35 or higher on the Economics roll. Likewise, reduce the treasury by 5% for every 3 that is needed to succeed (10% at 14, 15% at 11 and so on) down to a maximum reduction of 25% when rolling 5. Note that only the Economics roll has any effect on the treasury, but the Economics roll has no effect on the Public (see below).
As an additional optional rule, the increase of the treasury will also increase the DC for the Economics roll, and decrease of the treasury will also decrease the DC for the roll. How much the DC increases or decreases should be up to the DM.

Public
Public can be affected by the rolls made above. A successful roll gives a +1 bonus on the Public roll, with additional +1 for every 5 above the DC you get (+2 at 25, +3 at 30 and so on). Likewise, an unsuccessful roll will give you a -1 penalty and additional -1 penalty for every 5 that is needed for success (-2 at 15, -3 at 10 and so on). Some exceptions exist though:
Military: The public generally distrust a big military force. Success gives a penalty instead of bonus. If the country is in open war with another nation, as successful Military roll adds bonus as normal.
Medical: Healing is important to the nation and so gives you an additional +1 bonus on a successful roll. However, because it is so important, it gives an additional -1 penalty on an unsuccessful roll.
Economics: Successful Economics roll does not give you any bonus to the Public roll.
General: Certain actions taken might give additional bonus or penalty to the Public roll. Allowing Death Sentence may give a +1 bonus on the Justice roll, but will also apply -1 penalty on the Public roll. Erecting a monument adds +1 bonus to the Public roll but will give an equal penalty to the Economics roll. Increasing or Decreasing Taxes will give you bonus or penalty to your Economics roll but will apply equal penalty or bonus to your Public roll.

The Public roll
Unlike the other Political rolls, the Public roll uses Charisma instead of Intelligence. The Public roll is a Charisma roll with any bonus from successful Political rolls (see above for exceptions). Characters may substitute his Charisma modifier with ranks in Bluff, Diplomacy or Intimidate. A monarch may let the nation think that everything is alright, convince them that things will get better, or threat them to behave.
The Public roll uses the Diplomacy table found in the Player’s Guide on page 72. Bluff and Intimidate, however, can never make the Public Helpful regardless of how good the roll is. Getting the Public to be Friendly through the use of Bluff or Intimidate doesn’t actually make them “Friendly” but they will behave as such regardless.

Rebellion
Hostile Public will rebel against the monarchy or any other government. Rebellion can be resolved in few ways: Roleplaying it using standard rules, using mass combat rules, or using the political rules. During a rebellion, DC for Justice, Military, Transportation and Economics rolls (as well as any Political roll that the DM feels should suffer for it) increase by 10. The rebellion ends when the Public roll is good enough to get the public to rise from Hostile to Unfriendly or better.

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This concludes the Political system made by me. The good thing about it is that it gets the players involved in the political aspect of their kingdom (assuming they have one). They may have a damn lot of money, which they can increase at the risk of rebellion. The DM should also make note of any personal use of the treasury, and reduce it as is appropriate. Despite what the players might think, the kingdom's treasury should not be endless.