The following review written by one of the Software Informer contributors applies to version 2.6
Unity Web Player is a web browser add-on 3D player for Windows and Mac OS X. It is a player that plays content created with Unity on your web browser. The player was designed with 3D content playback in mind, so it is no wonder that it is the main use for the player.
However, it can still be used to play 2D content as well. Perhaps the most popular use of this player is to allow the user to interact with 3D character models to create personal avatars or in-game characters. I've recently used Unity Web Player on EA SPORTS website to create a game face. The player there showed a plain head which you could customize in many ways. You could add hair, facial hair, and modify facial features. And that was all done thanks to Unity. As a rule, for Unity Web Player to display anything, the content has got to be done with Unity, which is a separate application. Unity Web Player is used for the web games a lot. The nice feature about the games is that they can "cache" their game files on your hard drive so that you don't have to load everything every time you play a game. Unity Web Player supports all major web browsers and it is a really quick install.