C

right side of div

#wrapper{
    background:red;
    overflow:auto;
}

#c1{
   float:left;
   background:blue;
}

#c2{
    background:green;
    float:right;
}​

<div id="wrapper">
    <div id="c1">con1</div>
    <div id="c2">con2</div>
</div>​