SmartSystemMenu extends system menu of all windows in the system. It appends next custom items to menu:




TrayIt! is described as '- minimize to the system icon tray any program (Outlook Express, Notepad, Word, Internet Explorer, Sunbird, Thunderbird, Opera, OpenOffice etc.) and get more room in the Taskbar!' and is an app. There are more than 25 alternatives to TrayIt! for a variety of platforms, including Windows, Linux, PortableApps.com, File Explorer and Thunderbird apps. The best TrayIt! alternative is SmartSystemMenu, which is both free and Open Source. Other great apps like TrayIt! are RBTray, KDocker, SmartContextMenu and Actual Window Manager.
SmartSystemMenu extends system menu of all windows in the system. It appends next custom items to menu:




Allows almost any program to be minimized to the system tray by right clicking its minimize button.

Last update is from 2015 but the project has been forked:
RBTray Fork


KDocker will help you dock any application into the system tray. This means you can dock Openoffice, xmms, Firefox, Thunderbird, anything!
Just point and click. Works for all NET WM compliant window managers - that includes KDE, GNOME, Xfce, Fluxbox and many more.



SmartContextMenu adds a context menu to all windows in the system. This program is enhanced version of SmartSystemMenu. I hope it will be more convenient, because it supports all types of windows, including windows without a system menu.





RBTray by Benbuck, fork of RBTray , is a small Windows program that runs in the background and allows almost any window to be minimized to the system tray by:



4t Tray Minimizer is a lightweight but powerful window manager, which helps you to free up space on the desktop and the taskbar via the following actions:




eXtra Buttons lets you add up to 10 additional buttons to the title bar of each window, which allow you to minimize the window to the tray, keep it on-top, send it to the background, adjust the window's transparency and more.



DM2 provides several Windows enhancements that may help in every-day work. One of the nicest and most popular features of DM2 is minimizing windows to floating icons® (unique feature!) freeing both task bar and tray bar space.


Make all windows but the active one transparent.
Features:

Dims all windows except the active one.
Features:

PS Tray Factory allows you to take a full control over icons in the system tray area of the taskbar. With PS Tray Factory you can: hide icons from system tray into menu, change the order of icons in the system tray (sort tray), restore tray icons after Explorer.

Discontinued vs discontinued. No way.