Gnusto is a Mozilla application which lets you play the hundreds of games both modern and ancient available in the Z-machine format. It's still under development, but good enough to play many games already.
Although it should appear to behave in just the same way as any other Z-machine interpreter, Gnusto is actually a just-in-time compiler: it translates Z-machine instructions to JavaScript on the fly, so that subsequent executions of the same code will run at speeds close to that of pure JavaScript.
Gnusto's development is hosted at mozdev.