I have been playing League of Legends since last summer, and I really enjoy the game. For those of you who do not know anything about League of Legends(LOL), you control a powerful champion placed on a battlefield. Each side has armies of automated minions that try to push forward, with you and a team of champions turn the tides of battle.
This game is also free to play forever.
Riot Games does have to make money, and they do so with micro transactions. There are a lot of different champions, and they do not give access to all of them. If you want to "catch them all" you have to put forth the cash. There are ways to get champions by just playing well, each time you play a match you get points, but it is at a slow enough rate you have to spend small amounts of money.
This is a good business model because a lot of people I know don't mind spending a couple of dollars on a new skin for a champion they love, and they will keep doing this. And the level of entry is lowered by having the game free in the first place, opening up the game for people who would normally not spend the money.