basic JQuery tooltip plugin
example:

$(function(){
    $("span#tooltip").hover(function(){
        $(this).toolTip({text:"ToolTip example..."});
    },function(){
        $(this).toolTip("destroy");
    });
});

you can alter the CSS by including the properties along with the text, i.e.

{text:"ToolTip example...",css:{width:"100px"}}

its not as diverse as other tooltips but it serves the same purpose as most and is only 1kb when compiled

(function($){
    methods = {
        show: function(opts){
            c = $.extend({
                position:   "absolute",
                background: "#999",
                color:      "#333",
                width:      "auto",
                border:     "1px solid #666",
                borderWidth:    "thin 2px 2px thin"
            },opts.css);
            if (typeof opts.text=='object')
            {
                if(!opts.text.title)
                    return false;
                tt = 1;
                t = opts.text.title;
                opts.ut = opts.text;
                opts.text.title = null;
            }   else {
                    tt = 0;
                    t = opts.text;
            }
            this.data.toolTip = $.extend({id:"TTip"+Math.floor(Math.random()*10000),tx:t,t:tt},opts);
            $('<div id="'+this.data.toolTip.id+'">').text(t).css(c).appendTo('body');
            return $(this).bind('mousemove',methods.pos);
        },
        destroy: function(){
            $t = $(this).data.toolTip;
            if($t.text&&typeof $t.ut=='object')
                $t.ut.title = $t.tx;
            return $("#"+$t.id).remove().unbind('mousemove');
        },
        pos: function(e){
            ie = document.all?1:0;
            pX = ie?e.clientX+document.documentElement.scrollLeft:e.pageX;
            pY = ie?e.clientY+document.documentElement.scrollTop:e.pageY;
            return $("#"+$(this).data.toolTip.id).css({left: pX+"px",top: pY+10+"px"});
        }
    };
    $.fn.toolTip = function(method){return methods[method]?methods[method].apply(this,Array.prototype.slice.call(arguments,1)):methods.show.apply(this,arguments);}
})($);