jQuery - A library whose elegance vastly exceeds anything else on the market to the extent it has redefined the Javascript landscape, has excellent performance, came on the scene with superb documentation.

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.