One of the funny things about Parker Bros.' MONOPOLY game is that no two people seem to play it exactly the same way. The game comes with a rulebook, but surprisingly few people have ever read it: the game is predominantly taught by one person to another. Moreover, although Monopoly is ingenious and can be a lot of fun, it has some pretty quirky rules, and in my opinion is far from perfect. These facts, taken together with the game's massive popularity, have led to it having probably more variants than any other modern commercial game.
Many people find Monopoly trivial and boring, usually because they played it too often as children with too few players, and without enough deal-making. Deals are the soul of Monopoly. I find the best way to play it is to run a movie in the background and creatively negotiate whenever the chance arises, especially when it isn't your turn. If no deal is forthcoming, just keep the dice moving and see what happens. Play with variants--silly ones if you like. Above all, keep the game light, and laugh.
The following, in order from most important to least important, are the changes I like to make to the rules of Monopoly. Whenever I play, I see how many of them I can talk the other players into before we begin. ;-)
1. The rules say that when you bankrupt another player, you get all of his or her properties. This unbalances the game too much. I feel that eliminating another player should be its own reward. When a player is bankrupted, therefore, I suggest that you roll a die for each of his or her properties. On a 1, the property goes up for immediate auction. On a 2-5 it returns to the bank, and becomes unmortgaged if it was previously. On a 6 it goes to the player who bankrupted the eliminated player, or to the bank if the player was bankrupted by a payment to the bank. The bankrupting player still gets all of the bankrupted player's money, as well as the resale value for all of his or her buildings.
2. The rules prohibit players from granting each other loans. I dislike this rule, since it is the only impediment the game makes to free trade. Presumably, the reason for this rule is to keep players from keeping each other in the game by issuing loans at the last minute before bankruptcy. To me, this is just part of the diplomatic angle of the game. However, if it is determined that a player who cannot make a certain payment is going bankrupt, that player should not be allowed to trade or give anything to other players before leaving the game. (Thus, a player who is scrambling to make a payment can propose deals with other players to save himself/herself, but should not actually exchange money or properties [or should at least keep a record of this] if it is not clear that he or she can make the payment.)
3. Players should not be allowed to collect rent in Jail. This wil make Jail an undesirable place throughout the game, whereas in normal play, it becomes desirable to be in Jail after most of the properties are bought up, which strikes me as silly. By these rules, you may only pay to leave Jail on your own turn, but you may use a Get Out Of Jail Free card at any time. I also suggest that the cost to leave Jail should be raised to $75.
4. The rules say that you may build houses and hotels at any time, and must auction them off if multiple players want the last few. This rue is needlessly messy. I prefer to allow building only at the start of one's turn. Players may still sell them to the bank at half-price whenever they want.
5. The Free Parking house rule is a popular classic, and while it's true that it adds more chance to the game, I like it. It gives those who are losing a hope of catching up, for one thing. All payments made to Chance, Community Chest, Income Tax, Luxury Tax, and leaving jail are placed in the center of the board, collected when a player lands on Free Parking. This pool is seeded with a $50 bill at the beginning of the game and after anyone lands on Free Parking.
6. Players start with only $1000, not $1500. This counteracts the fact that the Free Parking rule puts more money into the game, and, more importantly, forces players to think about what they want to buy. With the actual $1500 starting amount, it's all too automatic to buy anything and everything you land on. Starting with less money adds strategy and puts more properties up for auction, which is fun.
7. The interest one pays to unmortgage a mortgaged property, 10%, is not very high. At that rate, there is little incentive not to mortgage unimproved properties, since they will not be bringing in much money anyway. I suggest playing with a mortgage interest rate of 50%. Thus, mortgaging a railroad will bring in $100 and will cost $150 to pay off later. I also suggest doing away with the rule that a player purchasing an unmortgaged property must pay a %10 penalty if he or she does not immediately unmortgage it.
8. According to the rules, if there are no houses available, you may not purchase hotels for properties with fewer than four houses. This creates a potential deadlock that adds nothing to the game. By my rules, hotels may be purchased directly, even if there are no houses available, by paying the price you would pay to first buy up to four houses and then to hotels.
9. According to the rules, you must sell all buildings on a property before trading or selling the property to another player. I find it more realistic and interesting to allow monopolies with buildings to be transfered without first selling the buildings. If a monopoly is split, of course, the buildings must be sold.
Also, for a way to spice up Monopoly, take a look at my list of Cosmic Monopoly Powers!