Google Related is a good opportunity for Google to track all the pages you visit and to offer something useful in return. After all, Google Related is a lot more useful than the PageRank button from Google Toolbar or Internet Explorer's Suggested Sites feature.
The bar is displayed at the bottom of the window, but not for all the pages. If there's not enough information related to the page, you won't see the bar. For example, Google Related is not displayed when you visit CNN's homepage, but it's displayed when you go to a CNN article. Another great thing about Google Related is that it shows various types of content, depending of the page you're visiting. Sometimes you'll see news articles and images, for other pages you'll find maps, local results and in other cases you'll see videos that play without leaving the page.
Google Related would be even better if you could see the related content on-demand (for example, by clicking a button). This way, you would no longer send your browsing history to Google and Google would get less information, so it's quite obvious why Google didn't add this option. Hopefully, Google will release an API for Google Related and other developers will create better extensions and widgets.
Update. Here an interesting quote from the book "I'm Feeling Lucky" by Douglas Edwards, who was Google's director of consumer marketing and brand management until 2005:
To tell you the PageRank of a site, Google needed to know what site you were visiting. The Toolbar sent that data back to Google if you let it, and Google would show you the green bar. The key was "if you let it," because you could also download a version of the toolbar that would not send any data back to Google. The user could make the choice, though Larry and the engineering team believed — and hoped — that most people wouldn't pass up the advanced features just because Google might learn their surfing habits. We're talking free extra data here. While knowing the PageRank of a page might have only nominal value to users, knowing the sites users visited would he tremendously valuable to Google. The PageRank indicator provided a justification for gathering it.
{ Thanks, Herin. }