Under Monopoly's standard rules, there are plenty of random penalties built into the game. For example, landing on Income Tax requires the player to pay a flat rate of $200 or 10 percent of his total worth. Landing on the Luxury Tax space charges the player a $100 fee. And landing on Chance or Community Chest makes the player draw a card that may result in a fine. Normally, all of these fees go to the bank.
Some house rules are built to offset those penalties. When players use the popular Free Parking jackpot rule, all the money collected from Income Tax, Luxury Tax, Chance and Community Chest goes to the center of the board instead of the bank (sometimes with a bonus $500 or more stuck in for good measure), and the first player to land on the Free Parking space gets all the loot. It's a pretty good payout for a space originally intended to have no action associated with it -- which is the whole problem.
Played by the book, Free Parking is a potential respite from the gambit of your opponents' highly developed properties. By giving the space a heavy monetary reward, you unbalance the natural cash flow of the game. The jackpot makes leading players richer, but it doesn't give straggling players enough of a boost to win -- just enough to prolong their loss (and the game). In the end, property ownership is what counts in Monopoly. All a losing player can do with that jackpot is feed it to his opponents. Unless you enjoy games that are both nigh-endless and frustrating, ditch this rule.