The week of May 26 to June 1 Oz Linden checked and reported that 1,665 different viewer versions that logged into Second Life™. That is an approximation of the number of different viewers in use. The SL system records viewer version strings. Each version of a viewer is to have a unique ID string. But, this is open source and people do whatever they do. So, there is a probability some ID strings don’t get changed. Thus I say approximate.

Some of these versions are one user only viewers, meaning: someone compiles a viewer for their own amusement and use. They never distribute the version. They may or may not change the version string.
Then there are the griefer viewers. They try to imitate other viewers and may duplicate another viewer’s version string to hide.
The result is going to be more viewer versions than the system recognizes.

