Can you believe that the treeview on the image below does not use any JavaScript, but relies only on CSS3? Check out this CSS3 TreeView, which is good with Twitter BootStrap:

You can get more info about this here http://acidmartin.wordpress.com/2011/09/26/css3-treevew-no-javascript/.

For those still searching for a tree with CSS3, this is a fantastic piece of code I found on the net:

http://thecodeplayer.com/walkthrough/css3-family-tree

HTML

<div class="tree">
<ul>
<li>
<a href="#">Parent</a>
<ul>
<li>
<a href="#">Child</a>
<ul>
<li>
<a href="#">Grand Child</a>
</li>
</ul>
</li>
<li>
<a href="#">Child</a>
<ul>
<li><a href="#">Grand Child</a></li>
<li>
<a href="#">Grand Child</a>
<ul>
<li>
<a href="#">Great Grand Child</a>
</li>
<li>
<a href="#">Great Grand Child</a>
</li>
<li>
<a href="#">Great Grand Child</a>
</li>
</ul>
</li>
<li><a href="#">Grand Child</a></li>
</ul>
</li>
</ul>
</li>
</ul>
</div>

CSS

* {margin: 0; padding: 0;}


.tree ul {
padding-top: 20px; position: relative;


transition: all 0.5s;
-webkit-transition: all 0.5s;
-moz-transition: all 0.5s;
}


.tree li {
float: left; text-align: center;
list-style-type: none;
position: relative;
padding: 20px 5px 0 5px;


transition: all 0.5s;
-webkit-transition: all 0.5s;
-moz-transition: all 0.5s;
}


/*We will use ::before and ::after to draw the connectors*/


.tree li::before, .tree li::after{
content: '';
position: absolute; top: 0; right: 50%;
border-top: 1px solid #ccc;
width: 50%; height: 20px;
}
.tree li::after{
right: auto; left: 50%;
border-left: 1px solid #ccc;
}


/*We need to remove left-right connectors from elements without
any siblings*/
.tree li:only-child::after, .tree li:only-child::before {
display: none;
}


/*Remove space from the top of single children*/
.tree li:only-child{ padding-top: 0;}


/*Remove left connector from first child and
right connector from last child*/
.tree li:first-child::before, .tree li:last-child::after{
border: 0 none;
}
/*Adding back the vertical connector to the last nodes*/
.tree li:last-child::before{
border-right: 1px solid #ccc;
border-radius: 0 5px 0 0;
-webkit-border-radius: 0 5px 0 0;
-moz-border-radius: 0 5px 0 0;
}
.tree li:first-child::after{
border-radius: 5px 0 0 0;
-webkit-border-radius: 5px 0 0 0;
-moz-border-radius: 5px 0 0 0;
}


/*Time to add downward connectors from parents*/
.tree ul ul::before{
content: '';
position: absolute; top: 0; left: 50%;
border-left: 1px solid #ccc;
width: 0; height: 20px;
}


.tree li a{
border: 1px solid #ccc;
padding: 5px 10px;
text-decoration: none;
color: #666;
font-family: arial, verdana, tahoma;
font-size: 11px;
display: inline-block;


border-radius: 5px;
-webkit-border-radius: 5px;
-moz-border-radius: 5px;


transition: all 0.5s;
-webkit-transition: all 0.5s;
-moz-transition: all 0.5s;
}


/*Time for some hover effects*/
/*We will apply the hover effect the the lineage of the element also*/
.tree li a:hover, .tree li a:hover+ul li a {
background: #c8e4f8; color: #000; border: 1px solid #94a0b4;
}
/*Connector styles on hover*/
.tree li a:hover+ul li::after,
.tree li a:hover+ul li::before,
.tree li a:hover+ul::before,
.tree li a:hover+ul ul::before{
border-color:  #94a0b4;
}

PS: apart from the code, I also like the way the site shows it in action... really innovative.

Another great Treeview jquery plugin is http://www.jstree.com/

For an advance view you should check jquery-treetable
http://ludo.cubicphuse.nl/jquery-plugins/treeTable/doc/

If someone wants vertical version of the treeview from Harsh's answer, you can save some time:

http://jsfiddle.net/Fh47n/

.tree li {
margin: 0px 0;


list-style-type: none;
position: relative;
padding: 20px 5px 0px 5px;
}


.tree li::before{
content: '';
position: absolute;
top: 0;
width: 1px;
height: 100%;
right: auto;
left: -20px;
border-left: 1px solid #ccc;
bottom: 50px;
}
.tree li::after{
content: '';
position: absolute;
top: 30px;
width: 25px;
height: 20px;
right: auto;
left: -20px;
border-top: 1px solid #ccc;
}
.tree li a{
display: inline-block;
border: 1px solid #ccc;
padding: 5px 10px;
text-decoration: none;
color: #666;
font-family: arial, verdana, tahoma;
font-size: 11px;
border-radius: 5px;
-webkit-border-radius: 5px;
-moz-border-radius: 5px;
}


/*Remove connectors before root*/
.tree > ul > li::before, .tree > ul > li::after{
border: 0;
}
/*Remove connectors after last child*/
.tree li:last-child::before{
height: 30px;
}


/*Time for some hover effects*/
/*We will apply the hover effect the the lineage of the element also*/
.tree li a:hover, .tree li a:hover+ul li a {
background: #c8e4f8; color: #000; border: 1px solid #94a0b4;
}
/*Connector styles on hover*/
.tree li a:hover+ul li::after,
.tree li a:hover+ul li::before,
.tree li a:hover+ul::before,
.tree li a:hover+ul ul::before{
border-color:  #94a0b4;
}

If someone wants expandable/collapsible version of the treeview from Vitaliy Bychik's answer, you can save some time :)

http://jsfiddle.net/mehmetatas/fXzHS/2/

$(function () {
$('.tree li').hide();
$('.tree li:first').show();
$('.tree li').on('click', function (e) {
var children = $(this).find('> ul > li');
if (children.is(":visible")) children.hide('fast');
else children.show('fast');
e.stopPropagation();
});
});

Building on Vitaliy's CSS and Mehmet's jQuery, I changed the a tags to span tags and incorporated some Glyphicons and badging into my take on a Bootstrap tree widget.

@import "../../../external/bootstrap/less/bootstrap.less"; /* substitute your path to the bootstrap.less file */
@import "../../../external/bootstrap/less/responsive.less"; /* optional; substitute your path to the responsive.less file */


/* collapsable tree */
.tree {
.border-radius(@baseBorderRadius);
.box-shadow(inset 0 1px 1px rgba(0,0,0,.05));
background-color: lighten(@grayLighter, 5%);
border: 1px solid @grayLight;
margin-bottom: 10px;
max-height: 300px;
min-height: 20px;
overflow-y: auto;
padding: 19px;
a {
display: block;
overflow: hidden;
text-overflow: ellipsis;
width: 90%;
}
li {
list-style-type: none;
margin: 0px 0;
padding: 4px 0px 0px 2px;
position: relative;
&::before, &::after {
content: '';
left: -20px;
position: absolute;
right: auto;
}
&::before {
border-left: 1px solid @grayLight;
bottom: 50px;
height: 100%;
top: 0;
width: 1px;
}
&::after {
border-top: 1px solid @grayLight;
height: 20px;
top: 13px;
width: 23px;
}
span {
-moz-border-radius: 5px;
-webkit-border-radius: 5px;
border: 1px solid @grayLight;
border-radius: 5px;
display: inline-block;
line-height: 14px;
padding: 2px 4px;
text-decoration: none;
}
&.parent_li > span {
cursor: pointer;
/*Time for some hover effects*/
&:hover, &:hover+ul li span {
background: @grayLighter;
border: 1px solid @gray;
color: #000;
}
}
/*Remove connectors after last child*/
&:last-child::before {
height: 30px;
}
}
/*Remove connectors before root*/
> ul > li::before, > ul > li::after {
border: 0;
}
}