Взято с http://snippets.su/topic/563/prikleennyy-footer-bez-hakov/.
html {
position: relative;
min-height: 100%;
}
body {
margin: 0 0 100px;
}
footer {
position: absolute;
left: 0;
bottom: 0;
height: 100px;
width: 100%;
}
<!DOCTYPE html>
<head>
<title></title>
</head>
<body>
<nav></nav>
<article>Lorem ipsum...</article>
<footer></footer>
</body>
</html>