The Cool VL Viewer is a third-party viewer for Second Life and OpenSim grids. In the name, "VL" of course stands for virtual life.
The exact reasons behind the creation of the Cool VL Viewer can be found in this message on the forum, but fact is that Linden Lab� (hereafter referred as by "LL") just cannot happen to take into account in a timely manner all the patches and bug fixes which the open source community provides. You can publish a patch and see it actually taken into account only months later... when at all !
Plus, LL often implements changes in their viewer without any or with very little regard to what their customers, the "residents" do like or not (one of the worst of all these changes in regard to the UI was probably the Voice viewers chatterbox, or the new sidebar in their v2.0 viewer UI). Such changes are considered by many users as true regressions, either in usability or functionality, and usually both.