PIN2TOP is a free AutoHotkey (AHK) script compiled to a tiny Windows executable. It is based on a very similar tool named "Always on Top" by Amit Agarwal, founder of the Digital Inspiration blog.
It has a new name, a more generic icon (to match the Windows 10 style better) and it doesn't limit itself to only a single line of code.
If you are curious what is in the script or if you want to build your own version check out the uncompiled script - or go straight to the download section if not.
First make sure pin2top.exe is running (start it manually or add it to the startup folder), then select a window and hit CTRL+SPACE to keep it always on top. If you want to unpin a window just do it again or exit PIN2TOP via the tray icon.