非常好的一个JS代码（RelativePosition.js）

var RelativePosition = function(){
    function getLeft( align, rect, rel ){
        var iLeft = 0;
        switch (align.toLowerCase()) {
            case "left" :
                return rect.left - rel.offsetWidth;
            case "clientleft" :
                return rect.left;
            case "center" :
                return ( rect.left + rect.right - rel.offsetWidth ) / 2;
            case "clientright" :
                return rect.right - rel.offsetWidth;
            case "right" :
            default :
                return rect.right;
        };
    };
    function getTop( valign, rect, rel ){
        var iTop = 0;
        switch (valign.toLowerCase()) {
            case "top" :
                return rect.top - rel.offsetHeight;
            case "clienttop" :
                return rect.top;
            case "center" :
                return ( rect.top + rect.bottom - rel.offsetHeight ) / 2;
            case "clientbottom" :
                return rect.bottom - rel.offsetHeight;
            case "bottom" :
            default :
                return rect.bottom;
        };
    };
    //定位元素 相对定位元素
    return function ( fix, rel, options ) {
        //默认值
        var opt = $$.extend({
            align:            "clientleft",//水平方向定位
            vAlign:            "clienttop",//垂直方向定位
            customLeft:        0,//自定义left定位
            customTop:        0,//自定义top定位
            percentLeft:    0,//自定义left百分比定位
            percentTop:        0,//自定义top百分比定位
            adaptive:        false,//是否自适应定位
            reset:            false//自适应定位时是否重新定位
        }, options || {});
        //定义参数
        var rect = $$D.clientRect(fix)
            ,iLeft = getLeft(opt.align, rect, rel) + opt.customLeft
            ,iTop = getTop(opt.vAlign, rect, rel) + opt.customTop;
        //自定义百分比定位
        if (opt.percentLeft) { iLeft += .01 * opt.percentLeft * fix.offsetWidth; };
        if (opt.percentTop) { iTop += .01 * opt.percentTop * fix.offsetHeight; };
        //自适应视窗定位
        if (opt.adaptive) {
            //修正定位参数
            var doc = fix.ownerDocument
                ,maxLeft = doc.clientWidth - rel.offsetWidth
                ,maxTop = doc.clientHeight - rel.offsetHeight;
            if (opt.reset) {
                //自动重新定位
                if (iLeft > maxLeft || iLeft < 0) {
                    iLeft = getLeft(2 * iLeft > maxLeft ? "left" : "right", rect, rel) + opt.customLeft;
                };
                if (iTop > maxTop || iTop < 0) {
                    iTop = getTop(2 * iTop > maxTop ? "top" : "bottom", rect, rel) + opt.customTop;
                };
            } else {
                //修正到适合位置
                iLeft = Math.max(Math.min(iLeft, maxLeft), 0);
                iTop = Math.max(Math.min(iTop, maxTop), 0);
            };
        };
        //加上滚动条
        iLeft += $$D.getScrollLeft(fix); iTop += $$D.getScrollTop(fix);
        //返回定位参数
        return { Left: iLeft, Top: iTop };
    };
}();

原文来自：

http://www.cnblogs.com/cloudgamer/archive/2009/10/29/Cloudgamer_JavaScript_Library.html

http://www.cnblogs.com/cloudgamer/archive/2009/08/10/FixedMenu.html