JavaScript Tree
Demo
Comments
The code has been tested on IE 4.0, 5.0, 5.5, 6.0 on Windows, IE 5.0 on Mac, and various versions of Netscape 6.0 and Mozilla.

Opera at this point does not have enough of the DOM for the tree to work.
Opera 5.10 to 5.12 for windows, Opera 5+ for Linux: 'style.display' can't be changed
Konqueror 2+: 'style.display' can't be changed