The complete list of Monopoly house rules. (via MeFi, which as a bonus links to your probability of landing on each square)

We always played with the following rules:

* Landing on Free Parking pays you $500 plus whatever money has been paid to Community Chest or in taxes
* Any agreed-upon 'deal' is legal, including immunity and interest-free loans
* Landing exactly on Go means $400, not $200
* You always have to pay rent, even if the property owner doesn't notice until after the next person rolls
* Players in jail can't collect rent

Occasionally we also played with the "doubling up" rule, except you got bounced to jail, not back to Go.

When we were very young my brother and I played with the "infinite hotels" rule, where you can build as many hotels as you want on a property, which gets very perverse very quickly. We stopped when we realized how stupid this was.

My father was always a big proponent of the "random start" feature, where a certain number of properties (and sometimes all of them) were shuffled and handed out at the start of the game, both to make the game go faster and to partially eliminate the didn't-go-first penalty. Negotiations can get very ugly in this version of the game, though.

Way better than Monopoly, by the way, was Solarquest. Here are the rules.