One visualisation method I keep coming back too is the treemap. Each time I experiment with them I learn a little bit more, but I usually end up abandoning them (with the exception of using quantum treemaps to display bibliographic data). But they keep calling me back.
My latest experiment builds on some earlier thoughts on quantum treemaps, but tackles two issues that have kept bugging me. The first is that quantum treemaps are limited to hierarchies that are only two levels deep (e.g., family → genus → species). This is because, unlike regular treemaps where you are slicing and dicing a rectangle of predetermined size, when you construct a quantum treemap you don't know how big it will be until you've made it (this is because you want to ensure that every item in the hierarchy can be displayed at the same size, and fitting them in may require you to tweak the size of the treemap). Given that taxonomic classifications have > 2 levels this is a problem. One approach is to construct quantum treemaps for the lower parts of the classification, then pack those into a larger rectangle. This is an instance of the packing problem. After Googling for a bit I came up across this code for packing rectangles, which was easy to follow and gave reasonable results.
The second problem is that I want the treemap to be interactive. I want to be able to zoom in and out and navigate around the treemap. After more Googling, I came across the Zoomooz.js library which makes web page elements zoom (for a pretty mind-blowing example of what can be done see impress.js), but I decided I want to work with SVG. After playing with examples from Keith Wood's jQuery SVG plugin I started to get the hang of creating zoomable visualisations in SVG.
Here's a video of what I've come up with so far (you can see this live at http://iphylo.org/~rpage/zoomrect/primates.html). This is an interactive display of the Catalogue of Life 2010 classification of primates, with images from EOL. It's crude, there are some obvious issues with redrawing images, labels, etc., but it gives a sense of what can be done. With care this could probably be scaled up to handle the entire Catalogue of Life classification. With a bit more care, it could probably be optimised for the iPad, which would be a fun way to navigate through the diversity of life.