test: http://jsdo.it/yksk/opyM

jquery.boxheight.js

$.fn.boxHeight = function() {
	var target = [],
		top = 0,
		height = 0;

	this.each(function() {
		var self = $(this);

		if (self.position().top != top) {
			target.length > 1 && $(target).height(height);
			target = [];
			height = 0;
			top = self.position().top;
		}
		target.push(this);
		height = Math.max(self.height(), height);
	});
	target.length > 1 && $(target).height(height);

	return this;
};