Our website uses TreeNodes to not only evaluate, but also to visualize the expression that is generated from user input.
It's not perfect, but our site _ actually _ works, and includes several other functions that were easily added as a byproduct of the Tree's structure, including conversion to prefix and postfix notations as well.
The CSS demonstrates a variety of fun features at the expense of tact and succinct web design - we know.
The biggest single challenge (not including adapting to CSS as we still haven't completely done that) was the recursive draw method we wrote, which displays a Tree of any size on the main section of the screen. This was difficult as we had to deal with the inevitability overlapping as best we could, by shrinking the leaves of each node with each layer of recursion while still maintaining readibility.