OK, time to put my money where my mouth is. Here's a first stab at displaying big trees in a browser. Not terribly sophisticated, but reasonably fast. Take a look at Big Trees. Approach Given a tree I simply draw it in a predetermined area (in these examples 400 x 600 pixels). If there are more leaves than can be drawn without overlapping I simply cull the leaf labels.

One of the striking pictures in Tamara Munzner et al.'s paper "TreeJuxtaposer: Scalable Tree Comparison using Focus+Context with Guaranteed Visibility" (doi:10.1145/882262.882291, also available here) is that of a biologist struggling to visualise a large phylogeny.