Here is a live demo of Pygmybrowse using the Catalogue of Life classification of animals provided by GBIF. It's embedded in this post in an tag, so you can play with it. Just click on a node. Taxa in bold have ten or more children, the numbers of children are displayed in parentheses "()". Each subtree is fetched on the fly from GBIF.

For the "to do" list, expand-ahead browsing looks like a useful approach to build upon PygmyBrowse (see my live demo). The approach is described in "Expand-Ahead: A Space-Filling Strategy for Browsing Trees" by McGuffin et al. (doi:10.1109/INFOVIS.2004.21, PDF also here). There is a video on Ravin Balakrishnan's site, which is an AVI file that I haven't bee able to coerce my Mac into playing, hence I've posted it on YouTube.