Google Wonder Wheel - There's a little known tool called the Wonder Wheel on every single Google search page. If you don't know it's there, it's hard to find. After you've searched for something on Google, you'll see a bold blue plus sign and a hyperlink that says "Show options..." right below Google's logo in the upper left. After you click that, a menu will slide out from the left side. The Wonder Wheel is near the bottom of this menu.
The wonder wheel is a visual, mind-mapping keyword tool. Whichever keyword you typed in the search box is at the center of the 8 spoke wheel. The 8 keywords that are the most closely related to that keyword (according to Google's algorithm) are at the end of each of the 8 spokes. When you click a keyword at the end of a spoke it zooms in on that one and shows you the 8 related keywords for that keyword. You can keep clicking and clicking to find related keywords.
