This is now included with Bootstrap 2.2.1.
Bootstrap 3.x
Use the navbar component and add .navbar-fixed-bottom
class:
<div class="navbar navbar-fixed-bottom"></div>
Bootstrap 4.x
<div class="navbar fixed-bottom"></div>
Don’t forget to add body { padding-bottom: 70px; }
or otherwise the page content may be covered.
Docs: http://getbootstrap.com/components/#navbar-fixed-bottom