My house Monopoly rules, adapted from rules my family has evolved and his excellent alternate rules roundup:
Rule: Buying property requires paying a finder's fee that effectively doubles the price.
Reason: Property is too inexpensive. In the normal game, it is always
a good strategy to purchase property. This way, players usually put
property up for auction unless they really want it. Players actually must think
about what they are willing to buy and when to start an auction.
Rule: The bank gets a token known at the "tax man" and a player will
roll the dice for the bank on its turn. If it lands on any owned
property, the owner pays the bank the amount that would normally be
charged in rent. The tax man ignores the normal meaning of any other
square, including "Go to Jail".
Reason: Balances the benefit of a property that often gets landed on
due to its placement on the board. Shortens the game. Adds drama.
Rule: When a player is bankrupted by debts to another player, the
debtor must mortgage all his or her properties, pay the creditor as
much as possible, and then return all of the debtor's properties to
the bank. These properties are then available for purchase by other
players as usual.
Clarifications: As with the normal rules, the debtor may not sell or trade any of his or her properties unless it gives enough money to pay the debt. But under this rule, no auction occurs of the lost property, and none of the property goes to any creditors or partners.
Reason: If a winning player gets a loser's property, they become a shoo-in to win. If there is an auction of the bankrupt properties, players with money will win. This way, all players have a chance.
Rule: Players may enter partnerships, but these carry risk.
Clarifications: In each partnership, one player must possess the entire monopoly and its houses. Partners may promise to split proceeds (and costs for houses), give free landings, and share investment, but neither partner may compel the other to buy houses. The owner may sell houses and mortgage the property, but must pay all other creditors before paying the partner half the proceeds. If a partner goes bankrupt, and they officially own the property, they must sell the houses and return the property to the bank. Terms of a partnership--including debts and free landings--transfer if the property is sold.
To smooth over these details, my family generally pre-negotiates a standard contract, which can be deviated from but by agreement but is in effect by default.
Reason: Stakes rise quickly because monopolies are nearly all built up, and more quickly. But the endgame can carry surprises because a powerful player with many partners can see their valuable property lost.
Rule: The two utilities cost double to land on: $8 per die roll for
one, $20 per die roll if both owned. This raises their average per-landing
value from $28 (one owned) or $70 (both owned) to $56 and $140, much more
Any new bid in an open auction must be at least $10 more than
the previous bid.
Free Parking Jackpot
$50 is placed in the middle of the board at the start of the
game. Any time a player must pay a fine or tax the money also is put
in the middle of the board. Whenever a player lands on Free Parking,
he or she gets the money in the middle. The $50 is replaced every time
a player lands there.
Reason: Throws another variable into the game and gives a losing
player a chance to get back into the game, or at least a dim hope.
Rule: A player in Jail may not collect rent, buy buildings,
participate in any auctions, or mortgage property.
Reason: Makes jail a punishment rather than a safe haven.