Second Life

Second Life is an online 3D virtual world imagined, created, and owned by its Residents that offers a platform for communication, business, education, and organizational development to anyone who wants to try it.

Minimum system requirements: 

CPU: 800 MHz (x86 compatible)
RAM: 512 MB
GPU: OpenGL 1.4 compatible, A display >=1024x768

External links:
Developer: Linden Lab