twitter-bootstrap
Bootstrap Affix

Su Navbar

html:

<nav class="navbar navbar-defalt" data-offset-top="120" data-spy="affix" >
    ...
</nav>

Css:

<style>
.navbar {
    background-color: red;
}
.navbar.affix {
    background-color: green;
}

</style>

Affisso Esempio 2

<div class="container" id="con">
    <div class="row">
        <div class="span12">
            <div class="well">
                <h1> Header </h1>               
            </div>
        </div>
    </div>
</div>
<div class="container" data-spy="affix" data-offset-top="400" id="nav">
    <div class="navbar">
        <div class="navbar-inner">
            <div class="container">
                <div class="span12">
                    <a class="brand" href="#">Home</a>
                     <ul class="nav">
                        <li class="active"><a href="#">Home</a></li>
                        <li><a href="#">Link</a></li>
                        <li><a href="#">Link</a></li>
                     </ul>
                </div>
            </div> 
        </div> 
    </div> 
</div>
<div class="container">
    <div class="span3">
        <p style="height:1000px;padding:10px;">
          
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        </p>
    </div>
</div>

<style>

#con .well {
    height:400px;
}

#nav.affix {
    position: fixed;
    top: 0;
    width: 100%
}
</style>

<script>
    $('#nav').affix();
</script