for Safari Plug-in allows you to search visually in all of your favorite search engines (Google, Yahoo, MSN, and countless others). Simply double-click on any word (or highlight any phrase), then plus and minus buttons will be displayed. You can press either button for the term to be added (or subtracted) to your current search and then it returns a new set of results based on your updated search. This extension really does save you vast amounts of time and is a great usability increase.
To see a 60 second video showing how this works please go here.