Why and what is the Cool VL Viewer ?
The "Cool VL Viewer"(*) (formerly known as "Cool SL Viewer" which first public release was v126.96.36.199, released on 2007-11-16) 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; the worst of all these changes in regard to the UI were probably the Voice viewers v1.18.2 chatterbox (and they did it again in v3 viewer's "Flexible UI", with the cumbersome and right out monstruous "Conversations" floater), or the 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.
As a response to the problems cited above, the Cool VL Viewer puts emphasis on high UI coherency from one version to the other (meaning no bad surprise for "old timers") while staying in sync with Linden Lab's official viewer features, high stability and reliability, and a high reactivity to new patches and bug fixes provided by the Open Source community. The Cool VL Viewer is also kept fully OpenSim-compatible.
(*) Please, get the name right when citing it elsewhere: the name is "Cool VL Viewer" (yes, "Viewer" is part of the name, just like "Tower" in "Eiffel Tower"), and not "Cool VL", "Cool Viewer", etc...
Who is maintaining this viewer ?
Henri Beauchamp (SL avatar name), creator of the "Cool Products" in SL.
For announcements and support, please see the Cool VL Viewer forum.
Why would I use your viewer ? What can I expect from it ?
Using this viewer or not is entirely up to you, of course. But it got quite a few bugs and regressions fixed, as well as really Cool new features. ...
The code changes used in this viewer have been either produced or reviewed and tested by me (and I have been programming for over 4 decades now... Not a script kiddy). This viewer is the one I use everyday, and should a patch introduce any regression, it would be either corrected or removed.
Such things as the actual usefulness of a new feature comes into consideration before implementing it. I try to stick the closest possible to LL's official viewer and only patch the bugs and regressions, sprinkling the mix with some, really useful features (such things I could not myself "live" without anymore after having tried them).
This viewer also implements the RestrainedLove (formerly known as "RestrainedLife") features: these features are disabled by default, so you won't notice any difference with a "normal" viewer till you enable them manually.
To enable or disable the new features of the Cool VL Viewer, you simply open the "Preferences" menu (which has been largely expanded and contains many new settings when compared with LL's viewers); be sure to read the tool tips for each setting (they are worth a full documentation).
Please note that since LL published a Third-Party Viewers (TPV) policy, in order to use this viewer to connect to Second Life, you must first read and agree with these terms (which will be presented to you on the first run of the Cool VL Viewer).
The Cool VL Viewer is itself TPV-policy compliant (and for people wondering why it is not listed in LL's TPV directory, it's simply because I don't want to provide private data to LL about myself, data which is illegal to require for such a purpose in my country. Being listed in the directory is in no way a requirement for a viewer to be considered TPV-policy compliant anyway: see the paragraph 6 of the TPV policy).
What operating systems and what hardware are supported ?
You will find Linux 64 bits and Windows 64 bits builds on this site, and links to MacOS X builds can be found in the Latest MacOS X Releases section of the forum (they have been built by Hyang Zhao, Guru Coyote and Kathrine Jansma in the past, and Catten Carter is now providing builds: thanks and kudos must go to them for their contribution).
The provided Linux viewer binary for the stable branch should run on any Linux 64 bits (2014-ish) system with glibc v2.19, libstdc++ v6.0.19 and glib v2.40.2 or newer. You may as well easily build the viewer yourself (with just one command !) on any Linux system (32 or 64 bits).
The provided Windows viewer binary should run on any Windows 64 bits system, from Windows Vista upwards. Instructions are also given in the sources tree for building 32 bits binaries with VS2013.
As for the hardware, you will obviously need a 64 bits CPU to run the provided builds. For 32 bits builds you would compile yourself, you will need a SSE2 capable CPU (which excludes Pentium 3 or Athlon XP CPUs, for example).
The supported video cards are the same as for the official Second Life viewers (i.e. OpenGL v2 capable and better graphic cards, with NVIDIA or ATI GPUs), but OpenGL v3+ capable hardware is highly recommended. Some Intel GPUs/APUs might fail to pass the requirements and are known to be too slow, most of the time.
For the memory, 8Gb of RAM is nowadays pretty much a minimum for a comfortable experience (you may thank the creators of the million-polys meshes, the 1024x1024 material textures (3 textures per face), and in general the poor design of most products uploaded in SL).
OK, where are the files ?
Please, see the Linux installation HOWTO or the Windows installation HOWTO for the details on how to install the viewer.
Google Chrome users, please also read this forum thread about the false and plain bogus malware warnings spewed by that lame browser !  You may also check the md5sum for the downloaded file if you wish.
Cool VL Viewer v1.26.22 (current stable branch):
Linux 64 bits: CoolVLViewer-188.8.131.52-Linux-x86_64-Setup (2019-01-12, 114Mb) to install, download the package, make it executable (chmod +x), then execute it and follow the dialogs.
Windows 64 bits: CoolVLViewer-184.108.40.206-Windows-x86_64-Setup.exe (2019-01-12, 90Mb) : to install, download the package then execute it and follow the dialogs.
MD5SUMs (*) for the current releases:26cff460c99f9335e46d729a72b224f2 CoolVLViewer-220.127.116.11-Linux-x86_64-Setup e428dcb7ae4eaae4b4b9f80c849823e3 CoolVLViewer-18.104.22.168-Windows-x86_64-Setup.exe 9c46985e0ed0e6d59553a327552eb3ad CoolVLViewer-src-1262230.tar.bz2
Linux users may use the 'md5sum' command from a terminal, or utilities such as gtkhash. Windows users may use any of the programs available to them; I'd personally recommend the HashCheck Shell Extension which is free, Open Source, integrates with the explorer (adding a "Checksums" tab to the file properties dialog), and works on all Windows versions (from XP to 10, 32 or 64 bits).