There are a couple of things you can do. First, simply adding the title attribute to the element you hover over will give you the little popup. For example, <a href="#" title="hello">link </a>.
The second thing you can do is create a div that contains a span that changes when hovered over. iow,
- <style type="text/css">
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span{visibility:hidden}
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a:hover span{visibility:visible}
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</style>
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</head>
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<body>
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<div><a href="#">boxy<span>hello</span></a></div>
An admittedly very crude examplel but the spanned element can be styled and you can get pretty creative with it.
The only reason you need an <a> is because IE doesn't :hover on anything else but all other browsers would allow a div:hover.