A computer. Maybe that sounds obvious to some people, but some of you are reading this article from your phone or a library computer. For serious web development, it’s better to invest in a desktop computer (Windows, Mac, or Linux).
Web browsers, to test code in. Currently the most-used browsers are Firefox, Chrome, Opera, Safari, and Internet Explorer. You should also test how your site performs on mobile devices and on any old browsers your target audience may still be using extensively (such as IE 6–8.)