Classic Shell is free software that improves your productivity, enhances the usability of Windows, and empowers you to use the computer the way you like it.Discontinued
The developer announced that Classic Shell will be no longer developed because Windows 10 is updated too frequently and because Microsoft is making more difficult to customize Windows: http://www.classicshell.net/forum/viewtopic.php?f=4&t=8147 Last version, 4.3.1, released in August 2017, can be still downloaded from the official website, together with the source code.
Classic Shell vs TranslucentTB opinions
Classic Shell has an option to customize the taskbar and make it opaque or totally transparent
TaskbarTools - Grants the ability to set the Windows Taskbar to any opacity and colour desired.Discontinued
The last version (1.0.8) was released in January 2017.