SquirrelFish Extreme (or SFX, for short) is the JavaScript engine of Safari. First announced in 2008, is now available in the latest versions of Safari. And it's the fastest JavaScript engine currently available: I've tested it and it's true. You can find more on this topic at http://webkit.org/blog/214/introducing-squirrelfish-extreme/.
Oh.....I thought this was about the fish, 'Squirrel Fish.'