I’m happy to announce one of my projects, js-sequence-diagrams. This uses Javascript to draw UML sequence diagrams in SVG format. Here is an example:
You can alter the diagram in real time, and I even have a simple jQuery plugin to make this easy to use on your own sites.
<script src="sequence-diagram-min.js"></script>
<div class="diagram">A->B: Message</div>
<script>
$(".diagram").sequenceDiagram({theme: 'hand'});
</script>