Código PHP:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title>Documento sin título</title>
</head>
<body>
<form id="form1" name="form1" method="post" action="" >
<select name="pais" onchange="(this.selectedIndex==this.options.length-1)?this.form.otropais.style.display='block':this.form.otropais.style.display='none'">
<option value="---------------"> --------------- </option>
<option value="Uruguay"> Uruguay </option>
<option value="Venezuela"> Venezuela </option>
<option value="Otro">Otro </option>
</select> <input type="text" name="otropais" size="18" id="otropais" style="display:none;"/>
</form>
</body>
</html>