Book whats? According to Wikipedia, a bookmarklet is:
a bookmark stored in a web browser that contains JavaScript commands to extend the browser's functionality. 
Sounds interesting, not? But is it useful? Yes it is! Personally I use them for a couple of often performed tasks, e.g:
On the sites mentioned above, the bookmarklets are shown as a button. You'll only have to drag them to the bookmarks bar in you browser. Otherwise you may create a new bookmark and copy the piece of code in it.

But that's only my selection. You'll find more inspiration on Lifehacker.com, MakeUseOf.com, or in Lifehacker: The Guide to Working Smarter, Faster, and Better by Gina Trapani & Adam Pash.

Do you use bookmarklets?