Windows: Even if you're the world's biggest Microsoft fan, you have to admit that Apple's "Quick Look" feature for macOS is pretty convenient.
If you're such a purist that you haven't even touched a Mac in the last decade or so, here's a brief introduction: You click on a file. You press the space bar. A preview of the item pops up - such as a photograph, the contents of a PDF, and so on.
It's a great way to take, well, a quick peek at something without wasting time loading an actual app.