With the inception of websites providing services it's become increasingly often that you're on one site and want to do something to it's content using the service of another site. The good news is that Bookmarklets can help you do this.
But how do Bookmarklets work?
Drag your language to the tool bar and press it whenever you need to translate a webpage.
Share in Reader
Instead of blogging about a blog you can just share it with your Google Reader friends.
Bookmark sites straight into your Google Bookmarks without having to install the Google Toolbar.
Make webpages more readable
Press the magic button and create text only versions of webpages.
Subscribe in Google Reader
Don't spend ages looking for the RSS button.
Shorten that URL
Create short URLs for Twitter and the like.
For more info check out the Bookmarklets website.