How to stretch a fixed number of horizontal navigation items evenly and fully across a specified container

Use text-align:justify on the container, this way it will work no matter how many elements you have in your list (you don't have to work out % widths for each list item

    #nav {
        text-align: justify;
        min-width: 500px;
    }
    #nav:after {
        content: '';
        display: inline-block;
        width: 100%;
    }
    #nav li {
        display: inline-block;
    }

<ul id="nav">
    <li><a href="#">HOME</a></li>
    <li><a href="#">ABOUT</a></li>
    <li><a href="#">BASIC SERVICES</a></li>
    <li><a href="#">OUR STAFF</a></li>
    <li><a href="#">CONTACT US</a></li>
</ul>

FIDDLE

This one really works. Also has the benefit that you can use media queries to easily turn off the horizontal style — for instance if you want to stack them vertically when on mobile phone.

HTML

<ul id="nav">
    <li><a href="#">Link</a></li>
    <li><a href="#">Link</a></li>
    <li><a href="#">Link</a></li>
    <li><a href="#">Link</a></li>
    <li><a href="#">Link</a></li>
    <li><a href="#">Link</a></li>
</ul>

CSS

​
#nav {
    display: table;
    height: 87px;
    width: 100%;
}

#nav li {
    display: table-cell;
    height: 87px;
    width: 16.666666667%;  /* (100 / numItems)% */
    line-height: 87px;
    text-align: center;
    background: #ddd;
    border-right: 1px solid #fff;
    white-space: nowrap;
}​

@media (max-width: 767px) {
    #nav li {
        display: block;
        width: 100%;
    }
}

http://jsfiddle.net/timshutes/eCPSh/416/

if you can, use flexbox:

<ul>
    <li>HOME</li>
    <li>ABOUT US</li>
    <li>SERVICES</li>
    <li>PREVIOUS PROJECTS</li>
    <li>TESTIMONIALS</li>
    <li>NEWS</li>
    <li>RESEARCH &amp; DEV</li>
    <li>CONTACT</li>
</ul>

ul {
  display: flex;
  justify-content:space-between;
  list-style-type: none;
}

jsfiddle: http://jsfiddle.net/RAaJ8/

Browser support is actually quite good (with prefixes an other nasty stuff): http://caniuse.com/flexbox