For example, Google Translation bookmarklet, when used will translate the current web page to the language of your choice.
"Get translations with a single click by adding one of the buttons below to your browser's toolbar. Then whenever you want to translate a webpage you're viewing, just click the button. You can translate any part of the page by selecting that part before you click.
To add a button, drag it from this page to your browser's toolbar"
Want more bookmarklets?
Here are some of my favorite bookmarklets: