Pure CSS tooltips by SantaKlauss shows you how to display tooltips using purely CSS. The idea is not unlike Nice Titles, though the execution is very different - Nice Titles uses the DOM to extract the nicetitles, the tooltips use only CSS.

The smart part is when printing: the tooltips are "inlined" and bracketed appropriately so that they show up in the normal flow of the document when printing.

Still, I consider the execution to be semantically incorrect since the tooltip requires the anchor tag <a> to work - the text that is "tooltipped" is not an anchor. A better way would have been to use a standard <span> tag. Of course, this then breaks in IE because IE only supports the :hover pseudo-class on the <a> tag. Thanks to whatever:hover, however, this can easily be fixed. Also, there seems to be no need for the z-index fix as mentioned (tested on IE/Win 5, 5.5 and 6). The CSS is below:

body {
behavior:url(/htc/csshover.htc);
}
span.info{
position:relative;
background-color:#ecc;
color:#000;
text-decoration:none
}
span.info:hover { background-color:#e96 }
span.info span.tooltip { display: none }
span.info:hover span.tooltip {
display:block;
position:absolute;
top:2em; left:2em; width:15em;
border:1px solid #0cf;
background-color:#cff;
color:#000;
text-align: center
}

Here's an example.