The easiest way to do this now is with a flexbox:
The CSS is then simply:
#container {
display: flex;
justify-content: space-between;
}
demo: http://jsfiddle.net/QPrk3/
However, this is currently only supported by relatively recent browsers (http://caniuse.com/flexbox).
Also, the spec for flexbox layout has changed a few times, so it’s possible to cover more browsers by additionally including an older syntax:
Using Flexbox: Mixing Old and New for the Best Browser Support