<div class="wrap">
...
<div class="footer">
Я футер
</div>
</div>
html, body {
min-width: 100%;
}
.wrap {
min-height: 100%;
position: relative;
padding-bottom: 100px;
}
.footer {
position: absolute;
bottom: 0;
left: 0;
width: 100%;
height: 100px;
}