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;
    });
}

I was looking at @jaider's solution which worked but I'm offering a slightly different approach that I think is more easy to work with and which I've seen to be good across browsers.

ul{
    list-style-type: disc;
    -webkit-columns: 2;
    -moz-columns: 2;
    columns: 2;
    list-style-position: inside;//this is important addition
}

By default un-ordered list display the bullet position outside but then in some browsers it would cause some display problems based on the browser's way of laying out your website.

To get it to display in the format:

A B
C D
E

etc. use the following:

ul li{
    float: left;
    width: 50%;//helps to determine number of columns, for instance 33.3% displays 3 columns
}
ul{
    list-style-type: disc;
}

This should solve all your problems with displaying columns. All the best and thanks @jaider as your response helped to guide me to discover this.

I tried posting this as a comment, but couldn't get the columns to display right (as per your question).

You are asking for:

A B

C D

E

... but the answer accepted as the solution will return:

A D

B E

C

... so either the answer is incorrect or the question is.

A very simple solution would be to set the width of your <ul> and then float and set the width of your <li> items like so

<ul>
    <li>A</li>
    <li>B</li>
    <li>C</li>
    <li>D</li>
    <li>E</li>
</ul>

ul{
    width:210px;
}
li{
    background:green;
    float:left;
    height:100px;
    margin:0 10px 10px 0;
    width:100px;
}
li:nth-child(even){
    margin-right:0;
}

Example here http://jsfiddle.net/Jayx/Qbz9S/1/

If your question is wrong, then the previous answers apply (with a JS fix for lacking IE support).

I like the solution for modern browsers, but the bullets are missing, so I add it a little trick:

http://jsfiddle.net/HP85j/419/

ul {
    list-style-type: none;
    columns: 2;
    -webkit-columns: 2;
    -moz-columns: 2;
}


li:before {
  content: "• ";
}

Here's a possible solution:

Snippet:

ul {
  width: 760px;
  margin-bottom: 20px;
  overflow: hidden;
  border-top: 1px solid #ccc;
}
li {
  line-height: 1.5em;
  border-bottom: 1px solid #ccc;
  float: left;
  display: inline;
}
#double li {
  width: 50%;
}

<ul id="double">
  <li>first</li>
  <li>second</li>
  <li>third</li>
  <li>fourth</li>
</ul>

And it is done.
For 3 columns use li width as 33%, for 4 columns use 25% and so on.