Firefox

If you've never tried it, you may just be in for a treat. Built by the Mozilla project. Their philosophy drives their innovation and popularity: "The Mozilla project is a global community of people who believe that openness, innovation, and opportunity are key to the continued health of the Internet. Since 1998, we’ve worked to ensure that the Internet is developed in a way that benefits everyone."

If you do any work with websites, you already know that the second-most popular browser has to be a part of the testing process. Further, the extensive set of add-ons give web developers access to tons of useful information about the way web pages are rendered.