Second Life (SL)
Second Life is a virtual world accessible via the Internet. A free program called the Second Life Viewer enables its users called Residents, to interact with each other through avatars (two-dimensional representations of people.) Residents can explore, meet other residents, socialize, participate in individual and group activities, and create and trade virtual property and services with one another, or travel throughout the world, which residents refer to as the grid. Second Life is for people aged 18 and over, while Teen Second Life is for people aged 13 to 17. (definition adapted from Wikipedia)
Among many other applications, SL is used to simulate situations in healthcare and teach clinical students in an almost real-world setting. In 2009, Medical Group Management Association (MGMA) used Second Life as part of its virtual conference.