In most browsers, the title attribute will render as a tooltip, and is generally flexible as to what sorts of elements it'll work with.
<span title="This will show as a tooltip">Mouse over for a tooltip!</span>
<a href="http://www.stackoverflow.com" title="Link to stackoverflow.com">stackoverflow.com</a>
<img src="something.png" alt="Something" title="Something">
All of those will render tooltips in most every browser.