What is an on-line Monopoly league anyway? For some reason, lot's of players are intimidated by these leagues. They're just groups of folks who play and socialize regularly. Leagues add a little spice to the game. People just create games, and play whoever happens to join. Others, make friends on-line and play with other friends. A league simply adds another dimension to it. League's maintain detailed statistics about who is playing whom, when, and who won. Leagues generally have a ranking system and players can compete in tournaments and one-on-one matches to move up in ranking.
Most leagues create special Monopoly rules. A popular example is “ultimate” Monopoly, where the only player permitted to purchase a property is the first one to land on that set. For example, the first player to land on, and purchase an orange property is the only one allowed to purchase the other orange properties. The result is that there are more monopolies formed, and more exciting play. I've heard of about 20 or 30 different variations created by these leagues. If you haven't tried a league, I'd strongly suggest you give it a shot. It's free, and it's rather addicting!
There are two on-line league sites, that I'm aware of, those are Casesladder/MyLeague and RankMonster. MyLeague is the most popular, and individuals can create and manage their own leagues. This article tells a bit about these leagues, and points out many that exist today.
Leagues are a great way to meet other folks. In fact, I've known several people who've actually hooked up and gotten married meeting in these leagues. I used to run a monopoly League on Playsite, when they hosted the game.
Here are some of the Leagues that exist today: