Different game worlds have different in-world currencies that can be traded on open markets.
They are seen as part of an emerging Virtual Economy
"According to figures collated by economics professor Edward Castronova, 2003 saw more than $9m of trades on Ebay category 1654 which covers internet games." (http://news.bbc.co.uk/2/hi/technology/3368633.stm)
Game currency markets
The Gaming Open Market "handles transactions in six separate currencies including Simoleons from The Sims Online, gold from Ultima Online and credits from Star Wars Galaxies. Soon it hopes to be helping people trade cash from Dark Ages of Camelot and EverQuest. Currencies are traded in blocks to make the market consistent and trade values easier to understand. The market makes its money by charging commission on deposits, withdrawals and trades."
Buy & Sell Game Money and Look Up Exchange Rates
Compiled by Greg Martin: