If you don't know what bookmarklets are, I recommend this introduction.
If you are familiar with bookmarklets but you are not sure how to install one, you can find here detailed instructions.
Bookmarklets are a powerwul way to automate tasks on the Web. In particular they can have significant security impact (see some of the examples below). The good news is that since they are written in JavaScript, it's pretty easy to review their code. For complete transparency, I show the formatted source code next to each of the bookmarklets below.