What is Second Life (SL)?

Second Life™ (SL) is an internet-based 3-D digital reality, founded by Linden Lab and created entirely by its residents. With over eleven million users, its own currency exchange, rapid growth since 2003 and vast business and entertainment opportunity, the landscape in this virtual medium is constantly evolving. Limited only by your imagination, one can experience, create, share, sell or buy anything in-world.

You can join with basic (free) membership, download the SL Viewer client, create your own avatar (virtual-self) and begin exploring immediately!

Resources:

1. Official Second Life website
2. Wikipedia: Second Life
3. Second Life in the News
4. Wikipedia: Businesses and Organizations operating in Second Life