How do I make my div 100% height?

You need to know what the 100% is of, so the parent div must have a height set. One problem that people often come up against is making the main page fill the screen if there's little content. You can do that like this :
CSS
body, html {
height:100%;
}
body {
margin:0;
padding:0;
}
#wrap {
position:relative;
min-height:100%;
}
* html #wrap {
height:100%;
}

Here, the #wrap div goes around your whole page - it's like a sub-body.

You need to use 'min-height' rather than 'height' for Firefox because otherwise it will set it to 100% of the viewport and no more. Internet Explorer, being well... crap, treats 'height' as it should be treating 'min-height' which it doesn't recognise. (You can target IE by preceding your code with ' * html ').

To make floated divs within this #wrap div 100% of the #wrap div... well that's more difficult. I think the best way is to use the 'faux columns' technique which basically means that you put the background in your body rather than your columns. If the body has columns and your floats don't then it looks like your floated content is in a column that stretches to the bottom of the page. I've used this technique in my layout demos.

The problem is often not that the columns aren't 100% height, but that they're not equal lengths. Columns usually don't start from the top of the page and end at the bottom - there's often a header and a footer or sometimes, more interesting designs don't have a recognisable columnar layout, but do require div boxes to be equal heights. This can be done with the aid of a couple of images and some css or with some javascript.