SEOQuake is a must. It is a “browser plugin” which means that you download it and install it for your preferred browser (easy and free).
Then, at the top of the every site you visit it will automatically provide some great information which will help you understand why Google ranks sites they way it does. The first four items alone are critical for SE rankings.
You can also click on each number to go to actually see the relative search engine’s collection of links.
You might want to disable SEOQuake when you are not going to use it because some SE’s (like Yahoo) will start returning an error message when it’s identified your computer as one that is repeatedly requesting this (free) information.
As far as browsers go, we recommend Google Chrome.