To get the widget, log-in to your Google Webmaster Tools and pick Tools -> “Enhance 404 pages” from the navigation. Now copy the code snippet you’ll get into your 404 template.
In the screenshot you can see a sample by Google showing the different features, and then a live test at this blog. I’ve activated the widget here for now (I’ll probably remove it later) so you can test it by entering some non-existing URLs at blogoscoped.com. Note the widget comes with some stylesheet classes so you can change the layout.
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