So you ended up with this JavaScript quirk where it was possible to create unique URLs that ran a bit of JavaScript on whatever page you happened to be looking at. It could even make changes to that page. Move things around. Replace words. Open links. And pretty early on, people realized that these JavaScript URLs were also bookmarkable, just like any other URL.

Source: Wait, what’s a bookmarklet?

Quick plug: I wrote a little thing to make something similar available on mobile more easily. Pure bookmarklets still work on every browser, but they’re not as ergonomic as they once were.