Forever I've used the macOS Command-Shift-4 screenshot utility to measure things. Pressing it gets you a little crosshairs cursor which you can click-and-drag to take a screenshot but, crucially, has little numbers that tell you the width/height of the selection in pixels. It's crude, but ever so useful.
PixelSnap is one of those apps that, once you see it, you're like OMG that's the best idea ever. It's the same kind of interaction (key command, then mouse around), but it's drawing lines between obvious measurement points in any window at all. Plus it has this drag around and area and snap to edges thing that's just as brilliant. Instant purchase for me.
The Product Hunt newsletter said:
Two teenage makers launched PixelSnap, a powerful design tool to measure every pixel on your screen. Hit #1 on Product Hunt, and over $5,000 in sales within 24 hours of their launch. 📝✨
Hey, even cooler!
A couple people pointed out xScope, which also has this feature. Fifty bucks, but also has a ton of other features. Tempting.