I created a web game called Pit of War (http://www.pitofwar.com) about three years ago and it has been generating enough monthly income to pay all my bills and affords me the ability to travel and live anywhere I like. It isn't completely passive but that is because I choose to add new features and updates. It is a niche game but the Internet is a big place with lots of people. :) Books like The Long Tail and The Curve have taught me that you don't need to have the #1 product in an industry to make a good living.
The game has two currencies. Hard currency (Trophies) that are mainly bought with real money and soft currency (Gold) which are found in game. It is a fairly standard practice for those familiar with Free-2-Play (F2P) games. It was difficult to balance things so paying players and free players could compete on level footing but after three years I feel I've found a nice balance and if judging from player feedback the majority also feels the same.
The game's forum uses advertising but the income is negligible, maybe around $10 per month. This is likely due to the fact that it is the same eyeballs looking at the ads and it is not a high traffic forum.
