Javascript
Javascript has become such an important language over the last few years. It has significantly altered the way webpages are created and used. Things like Google Maps would never have been a possibility.
Things like JQuery, Prototype, and YUI have made building "Web 2.0" applications so much easier, and web browsers are begining to seriously focus on Javascript execution times.
I've also been able to work with it while building Firefox applications, which use it to manipulate the Firefox DOM.