Oh now, baby, this is good: the Tree of Life Web Project (beta). Tree view at each level (as a Java applet), copious links to more information and papers, doesn't use the taxonomy I learnt at school but a more flexible groups and leaves system. Man oh man. The technical description hints at open sourcing the whole thing in the future (the data too? Please!), but the coolest thing is navigating the tree to find, say, Homo sapiens -- start from the Root of all Life and work your way along. Brilliant.