Your need to write the function with an argument !
<html>
<head>
<script>
function content([COLOR="#FF0000"]that[/COLOR]){
alert ([COLOR="#FF0000"]that[/COLOR].innerHTML);
}
</script>
</head>
<body>
<div onclick='content(this)'>some text here</div>
</body>
</html>
An other solution consist to call the arguments array (here arguments[0]) in the function.