其实现代浏览器大多都支持文本框尺寸调节功能，绝大多数情况下却没有自动适应来得爽快，在网络上发现一方法比较简单的实现文本框高度自适应，于是封装了这个函数，准备以后应用到项目中。
源代码：
23:03文章更新：
感谢alucelx同学再次给力的帮助，大大简化了方法，更新代码为0.2版本，同时解决了兼容Opera浏览器，至此全兼容IE6+与现代浏览器！
在线演示： http://demo.jb51.net/js/2011/autoArea/index.htm
autoTextarea.js
代码如下:
/**
* 文本框根据输入内容自适应高度
* @author tang bin
* @version 0.3
* @see http://www.planeart.cn/?p=14
* @param {HTMLElement} 输入框元素
* @param {Number} 设置光标与输入框保持的距离(默认20)
* @param {Number} 设置最大高度(可选)
*/
var autoTextarea = function (elem, extra, maxHeight) {
extra = extra || 20;
var isFirefox = !!document.getBoxObjectFor || 'mozInnerScreenX' in window,
isOpera = !!window.opera && !!window.opera.toString().indexOf('Opera'),
addEvent = function (type, callback) {
elem.addEventListener ?
elem.addEventListener(type, callback, false) :
elem.attachEvent('on' + type, callback);
},
getStyle = elem.currentStyle ? function (name) {
var val = elem.currentStyle[name];
if (name === 'height' && val.search(/px/i) !== 1) {
var rect = elem.getBoundingClientRect();
return rect.bottom - rect.top -
parseFloat(getStyle('paddingTop')) -
parseFloat(getStyle('paddingBottom')) + 'px';
};
return val;
} : function (name) {
return getComputedStyle(elem, null)[name];
},
minHeight = parseFloat(getStyle('height'));
elem.style.maxHeight = elem.style.resize = 'none';
var change = function () {
var scrollTop, height,
padding = 0,
style = elem.style;
if (elem._length === elem.value.length) return;
elem._length = elem.value.length;
if (!isFirefox && !isOpera) {
padding = parseInt(getStyle('paddingTop')) + parseInt(getStyle('paddingBottom'));
};
scrollTop = document.body.scrollTop || document.documentElement.scrollTop;
elem.style.height = minHeight + 'px';
if (elem.scrollHeight > minHeight) {
if (maxHeight && elem.scrollHeight > maxHeight) {
height = maxHeight - padding;
style.overflowY = 'auto';
} else {
height = elem.scrollHeight - padding;
style.overflowY = 'hidden';
};
style.height = height + extra + 'px';
scrollTop += parseInt(style.height) - elem.currHeight;
document.body.scrollTop = scrollTop;
document.documentElement.scrollTop = scrollTop;
elem.currHeight = parseInt(style.height);
};
};
addEvent('propertychange', change);
addEvent('input', change);
addEvent('focus', change);
change();
};

测试代码：