How do I justify a horizontal list?

You need to use a "trick" to make this work.

See: http://jsfiddle.net/2kRJv/

HTML:

<ul id="Navigation">
    <li><a href="About.html">About</a></li>
    <li><a href="Contact.html">Contact</a></li>
    <!-- ... -->
    <li class="stretch"></li>
</ul>

CSS:

#Navigation
{
    list-style-type: none;
    text-align: justify;
    height: 21px;
    background: #ccc
}

#Navigation li
{
    display: inline
}
#Navigation .stretch {
    display: inline-block;
    width: 100%;

    /* if you need IE6/7 support */
    *display: inline;
    zoom: 1
}

Details on IE6/7 trickery: Inline block doesn't work in internet explorer 7, 6

Just do:

ul { width:100%; }
ul li {
  display:table-cell;
  width:1%;
}

Modern Approach - CSS3 Flexboxes

Now that we have CSS3 flexboxes, you no longer need to resort to tricks and workarounds in order to make this work. Fortunately, browser support has come a long way, and most of us can start using flexboxes.

Just set the parent element's display to flex and then change the justify-content property to either space-between or space-around in order to add space between/around the children flexbox items. Then add additional vendor prefixes for more browser support.

Using justify-content: space-between - (example here):

ul {
    list-style: none;
    padding: 0;
    margin: 0;
}
.menu {
    display: flex;
    justify-content: space-between;
}

<ul class="menu">
    <li>About</li>
    <li>Contact</li>
    <li>Contact Longer Link</li>
    <li>Contact</li>
</ul>

Using justify-content: space-around - (example here):

ul {
    list-style: none;
    padding: 0;
    margin: 0;
}
.menu {
    display: flex;
    justify-content: space-around;
}

<ul class="menu">
    <li>About</li>
    <li>Contact</li>
    <li>Contact Longer Link</li>
    <li>Contact</li>
</ul>

This can also be achieved using a pseudo element on the ul element:

ul {
    margin: 0;
    padding: 0;
    list-style-type: none;
    text-align: justify;
}

ul:after {
    content: "";
    width: 100%;
    display: inline-block;
}

li {
    display: inline;
}