An excellent example of this technology is this Google News bookmarklet (copy this short cut into your browser as a bookmark, and then start using it!) detailed here on Danny O' Brien's Oblomovka blog. Just add the bookmarklet to your browser's favourites, and when exploring news sites, click on the bookmarklet to see Google News' list of related stories.
The bookmarklets.com web site contains many more bookmarklets, all free for use - and modification.
Now why aren't there any Live Journal bookmarklets out there?