Second Life is an online virtual world, developed and owned by the San Francisco-based firm Linden Lab and launched on June 23, 2003.
IMVU is a 3D based social network using avatars, with user created rooms and high levels of avatar and room customisation. (Room creation requires the PC or Mac client).
In Facebook Horizon, not only can you explore the virtual world as it grows and changes, but you can discover all sorts of new things that interest and inspire you. Experience gravity defying, distance spanning, genre bending games of all types.