Here’s one for those who use Apple’s Notes app for storing bits of information. By default, Notes in macOS gives you a single window, with each note listed in a sidebar.

But what if you want to see two notes at once? Or keep one always available no matter what else you’re doing? Select the desired notes in the sidebar by Command-clicking them, and then choose Window > Float Selected Notes to open them in their own windows.

Or, just double-click them in the sidebar! Then, to make sure one or more of those windows is never obscured by another app, make it active and then choose Window > Float on Top. It’s still a normal window that you can move and resize and close, but no other app will appear over it. See how Safari is the frontmost app below, but the Notes window is on top?