function getSelect() {
    var factor = document.getElementById("hi");
    var fa = factor.selectedIndex;
    var username = factor.options[fa].value;
    if (username != '请选择') {
        window.open(username, '');
    }
}

document.write("21世纪中国不动产体系网站：<select name='hi' onchange='getSelect();'  id='hi' >");
document.write("<option value='请选择'>请选择</option>");
document.write("<option value='http://www.century21cn.com'>21世纪中国不动产</option>");
document.write("<option value='http://www.c21ax.com'>北京安信</option>");
document.write("<option value='http://www.c21ruifeng.com'>上海锐丰</option>");
document.write("<option value='http://www.c21sz.com'>深圳泛城</option>");
document.write("<option value='http://www.c21chengdu.com.cn'> 成都亿泽</option>");
document.write("<option value='http://www.xunloan.com'>北京凯盛 </option>");
document.write("<option value='http://www.xunloan.sh.cn'>上海凯盛</option>");
document.write("<option value='http://shenzhen.xunloan.com'>深圳凯安</option>");
document.write("<option value='http://www.c21shy.com'>商业地产</option>");
document.write("<option value='http://www.shanggu.com.cn'>上古地产</option>");
document.write("<option value='http://www.c21xlp.com/'>新楼盘（凯诚华信）</option>");
document.write("</select>");