How to fill 100% of remaining width

For anyone looking over this now theres a newish css property method called calc which can perform this in a much more flexible fashion.

<div class="container">
    <div class="fixedWidth"></div>
    <div class="variableWidth"></div>
</div>

.fixedWidth{
width:200px;
}
.variableWidth{
width:calc(100%-200px);
}

As a word of warning, this is not very portable and support is ropey on mobile devices. IOS 6+ and andriod 4.4 i believe. Support is significantly better for desktop though, IE 9.0+.

http://caniuse.com/calc

I have used a JS hack in the past to achieve this technique if anyone is incredibly stuck, a different layout is more advisable though as resize is slower.

window.addEventListener('resize', function resize(){
    var parent = document.getElementById('parent');
    var child = document.getElementById('child');
    child.style.width = parseInt(parent.offsetWidth - 200) + "px"; //200 being the size of the fixed size element
}, false);

If you don't know how big will be the fixed part you can use the flex 9999 hack.

<div class="container">
  <div class="fixedWidth"></div>
  <div class="variableWidth"></div>
</div>

.container {
    display: flex;
    flex-direction: row;
    flex-wrap: wrap;
}

.fixedWidth {
    flex: 1;
}

.variableWidth {
    flex: 9999;
}

Updated answer:

The answers here are pretty old. Today, this can be achieved easily with flexbox:

.container {
  border: 4px solid red;
  display: flex;
}
.content {
  border: 4px solid green;
  flex-grow: 1;
  margin: 5px;
}
.sidebar {
  border: 4px solid blue;
  margin: 5px 5px 5px 0;
  width: 200px;
}

<div class="container">
  <div class="content">
    Lorem ipsum dolor sit amet.
  </div>
  <div class="sidebar">
    Lorem ipsum.
  </div>
</div>

Original answer:

Block level elements like <div> will fill 100% of the available width automatically. If you float one of them to the right, the contents of the other will fill the remaining space.

<div style="height: 100px; border: 3px solid red;" id="container">
  <div style="float: right; width: 100px; border: 3px solid blue;">Fixed</div>
  <div style="border: 3px solid green;">Fill</div>
</div>

http://jsfiddle.net/5AtsF/