How to display an unordered list in two columns?

Modern Browsers

leverage the css3 columns module to support what you are looking for.

http://www.w3schools.com/cssref/css3_pr_columns.asp

CSS:

ul {
  columns: 2;
  -webkit-columns: 2;
  -moz-columns: 2;
}

http://jsfiddle.net/HP85j/8/

Legacy Browsers

Unfortunately for IE support you will need a code solution that involves JavaScript and dom manipulation. This means that anytime the contents of the list changes you will need to perform the operation for reordering the list into columns and reprinting. The solution below uses jQuery for brevity.

http://jsfiddle.net/HP85j/19/

HTML:

<div>
    <ul class="columns" data-columns="2">
        <li>A</li>
        <li>B</li>
        <li>C</li>
        <li>D</li>
        <li>E</li>
        <li>F</li>
        <li>G</li>
    </ul>
</div>

JavaScript:

(function($){
    var initialContainer = $('.columns'),
        columnItems = $('.columns li'),
        columns = null,
        column = 1; // account for initial column
    function updateColumns(){
        column = 0;
        columnItems.each(function(idx, el){
            if (idx !== 0 && idx > (columnItems.length / columns.length) + (column * idx)){
                column += 1;
            }
            $(columns.get(column)).append(el);
        });
    }
    function setupColumns(){
        columnItems.detach();
        while (column++ < initialContainer.data('columns')){
            initialContainer.clone().insertBefore(initialContainer);
            column++;
        }
        columns = $('.columns');
    }

    $(function(){
        setupColumns();
        updateColumns();
    });
})(jQuery);

CSS:

.columns{
    float: left;
    position: relative;
    margin-right: 20px;
}

EDIT:

As pointed out below this will order the columns as follows:

A  E
B  F
C  G
D

while the OP asked for a variant matching the following:

A  B
C  D
E  F
G

To accomplish the variant you simply change the code to the following:

function updateColumns(){
    column = 0;
    columnItems.each(function(idx, el){
        if (column > columns.length){
            column = 0;
        }
        $(columns.get(column)).append(el);
        column += 1;
    });
}