For example, if I search for 'Eclipse' while wearing the 'Software Developer' hat, I should get Eclipse IDE related links before links related to the astrophysical phenenomon. If even I was interested in the later, results I get back should be different depending on whether I am wearing a Physicist's hat or a Photographer's hat.
That example is rather silly, though. If you're searching for the Elipse IDE, obviously you can just search for eclipse ide or eclipse java development, and if you're searching for astronomical eclipses, you'd search for something like lunar eclipse.
What would be useful is an "I'm not shopping for anything" option. There are many sites that get in the way in Google's search results when you're trying to research certain consumer electronics, for example.
The state is that great fiction by which everyone tries to live at the expense of everyone else. - Frederic Bastiat