Gamers have been making money from games for years but most of them did it by breaking the game's Terms of Agreement. Probably the most known game for "Farmers" is World of Warcraft, the web is full of ads with offers for gold and power-leveling services, the problem is that it's against Blizzard policy and not only you can get you account banned but most of the time you will probably get scammed.
There are very few games that allow players to trade their virtual currency for real money, and even fewer that encourage it. Real Cash Economy Games are not exactly new, some of them were here long before World of Warcraft and other games like it, but they are not every known even thought virtual items in these games were sold for hundreds of thousands or real USD. The pioneers to RCE games are Second Life from Linden Lab and Project Entropia (now known as Entropia Universe) developed by MindArk.
Second Life was launched in June, 2003 and since than, according to Wikipedia, more than 21.300.000 accounts were created, although the company - Linden Lab - has not made public figures for long-term consistent usage. Second life is Free to play but offers premium memberships as well, but most players don't upgrade beyond the basic, free, membership.
According to figures published by Linden Lab in February, 2009 over 60.000 users made a profit in Second Life ,of whom ~40.000 made less than 10$, while 233 made over 5.000$. In March, 2009 it was revealed that there are a few successful gamers who made over 1.000.000$ USD per year.
Entropia Universe (formerly known as Project Entropia) is another RCE MMORPG giant created by MindArk a Swedish company. Just like Second Life, Entropia is free to play but it uses a micro-payment system, where players can buy in-game currency (PED - Project Entropia Dollars) with real money that can be converted back at anytime at a fixed rate of ten to one, that means that any in-game item has a real cash value.
In 2004 and 2008, it entered the Guinness World Record for the most expensive virtual item sold, and then in 2009 when a virtual property was sold for 330.000$, a record that was beaten in 2010 when Jon Jacobs sold a Club NeverDie for 635.000$. According to Wikipedia in June, 2010 the Swedish company released its 2009 annual report. They reported a transaction total of over 2.400.000$ USD. MindArk said that they sold their first planet - Planet Calypso - for 6.000.000$ to "See Virtual Worlds"
Two newcomers are Anno1777 and ProEconomica browser-based games, Anno1777 has already made payments of thousands of Euro to a total of over 200.000 Euro. Both games are financial and economical simulators.