Windows only: Freeware application Trayconizer turns any application into a system tray application in just a few steps. Unlike other previously mentioned minimize-to-system-tray apps like TrayIt or 4t Tray Minimizer, Trayconizer starts applications in the system tray from the moment you run them. To use it, you need to tweak the shortcut you’re using to launch the application (right-click shortcut and select Properties) by adding the path to Trayconizer in front of the app like so:
C:\Path\To\Trayconizer.exe C:\Windows\Notepad.exe. Trayconizer is freeware, Windows only, requires no install, and consumes a paltry 2MB of RAM.