CSS fill remaining width

Use calc!

HTML:

<div class="left">
  100 px wide!
  </div><!-- Notice there isn't a space between the divs! *see edit for alternative* --><div class="right">
    Fills width!
  </div>

CSS:

.left {
  display: inline-block;
  width: 100px;

  background: red;
  color: white;
}
.right {
  display: inline-block;
  width: calc(100% - 100px);

  background: blue;
  color: white;
}

Update: As an alternative to not having a space between the divs you can set font-size: 0 on the outer element.