SecondLife from Linden Labs has been one of the most successful shared virtual environment platforms. Although not the first by any means, it has attracted many users. At UCL we have used it for various purposes over the years.
SecondLife was architected in a period when the performance of the target computers was considerably lower. In particular, the graphics performance of machines is much higher. Unfortunately immersive virtual reality is now pushing even the best graphics engines to their limits and the SecondLife engine cannot cope.
Linden Labs have announced a new platform codenamed Project Sansar. Details are a little thin on the ground at the moment, but you can listen to an interview with Linden Lab CEO Ebbe Altberg on the excellent Voice of VR podcast.