I'll throw jsTree into the ring, too. I've found it fairly adaptable to your particular situation. It's packed as a jQuery plugin.
It can run from a variety of data sources, but my favorite is a simple nested list, as described by @joe_coolish or here:
<ul>
<li>
Item 1
<ul>
<li>Item 1.1</li>
...
</ul>
</li>
...
</ul>
This structure fails gracefully into a static tree when JS is not available in the client, and is easy enough to read and understand from a coding perspective.