I got back into bookmarklets recently, when I needed a way to hide YouTube's full-screen UI. Normally, it slides out of view a few seconds after you hit play, but that makes it difficult to take screenshots at precise times.
A quick Google search turned up this bookmarklet:
Transforming a code snippet into a bookmarklet in Chrome is incredibly easy -- just highlight the script's text and drag it onto the bookmarks bar (after which, you'll want to rename it to something descriptive).
But bookmarklets don't have to be straightforward; you can do some pretty complex stuff with them these days. Here's one that lets you examine the layers of a webpage in 3D, courtesy of developer Ted Michael Celis:
How about an interactive piano? Celis has you covered there as well:
Bookmarklets can even let you extend the functionality of built-in features. Case in point: Brendan Sechter created a few to alter the playback speed of YouTube videos beyond what the UI allows.
If you're interested in learning more about bookmarklets, Mozilla has a great article that explains the ins and outs.