I use this AutoHotkey oneliner. WIN+Space pins/unpins the current window.
#SPACE:: Winset, Alwaysontop, , A
(Save to set-always-on-top.ahk, and create a shortcut to the script in %userprofile%\AppData\Roaming\Microsoft\Windows\Start Menu\Programs\Startup. Optionally add #NoTrayIcon at the top of the script to get rid of the tray icon.)