Firebug – The tool that increased my web programming productivity by a factor of 3.
Web Developer Toolbar – Before Firebug, there was Web Developer Toolbar, an excellent way to inspect the page state that is still useful at times.
The Ajax Design Pattern – Yes, JJG didn’t come up with anything new and people were doing it before, but this was a “right time, right place” case where the name immediately spawned a massive community of people sharing ideas and experiences.
Firefox – Browser + Detailed Settings + Tabs (credit Opera) + Plugin Mechanism = developer’s best friend.
View Source – The ability to look under the covers, responsible for rapid accumulation of knowledge in web apps.
StackOverflow – Increasingly becoming the no-brainer search result for programming problems and a vast improvement over the cruddy SEOtastic train wrecks that went before it.