ECMAScript 5
When I started working on Caja I had not really plumbed the depths of Javascript (or, as it is more correctly called, ECMAScript 3) and I was very surprised to learn how powerful it actually is. I was also pretty startled by some of the nasty gotchas lurking for the unwary (or even wary) programmer (had I known, perhaps I would never had tried to get Caja off the ground!).
For some time now, the ECMAScript committee has been working on a new version of Javascript which fixes many of these problems without breaking all the existing Javascript that is out there. This seems to me a remarkable achievement; Mark Miller, Mike Samuel (both members of the Caja team) and Waldemar Horwat gave a very interesting talk about these gotchas and how the ES5 spec manages to wriggle around them. I recommend it highly. Slides are available for those who don’t want to sit through the presentation, though I would say it is worth the effort.