Second Life is a virtual world, and the Second Life Viewer is the official application used to enter it. Using the viewer you can logon and off, view, move about in, and hear the virtual world.
The Firestorm Project is a viewer for Second Life and OpenSimulator, and the successor to the now discontinued Phoenix Viewer. Firestorm Viewer is based on the Linden Lab V3 LGPL code base and has an enormous number of features, options and interface customization choices...
Singularity Viewer is an exciting client for Second Life and OpenSim, which strives to combine the beloved look and feel of Viewer 1.23 with the latest and greatest of available technology, and to stay compatible with future Second Life changes and features.
Kokua is an open source viewer for Second Life and OpenSim-based virtual worlds. The goal of Kokua is to improve the user interface and usability of the viewer through community involvement, thoughtful design, modern development methods, and a pro-change atmosphere.