This cross-browser lib seems safer - just zoom and moz-transform won't cover as many browsers as jquery.transform2d's scale().
http://louisremi.github.io/jquery.transform.js/
For example
$('#div').css({ transform: 'scale(.5)' });
OK - I see people are voting this down without an explanation. The other answer here won't work in old Safari (people running Tiger), and it won't work consistently in some older browsers - that is, it does scale things but it does so in a way that's either very pixellated or shifts the position of the element in a way that doesn't match other browsers.
http://www.browsersupport.net/CSS/zoom
Or just look at this question, which this one is likely just a dupe of: