As announced on phylobabble I've started to revisit visualising large phylogenies, building on some work I did a couple of years ago (my how time flies). This time, there is actual code (see https://github.com/rdmpage/deep-tree) as well as a live demo http://iphylo.org/~rpage/deep-tree/demo/.
You can see the amphibian tree below at http://iphylo.org/~rpage/deep-tree/demo/show.php?id=5369171e32b7a:
You can upload or paste a tree (for now in NEXUS format), or paste in a URL to a NEXUS file (e.g., from TreeBASE). I'll formats when I get the chance. The viewer uses the same approach as Google Maps, breaking the image of the tree into "tiles" of a fixed size, so even if the tree image is huge, the web browser only ever displays the same number of tiles. You can zoom in to see individual taxa, or zoom out for an overview. One reason I'm building this is to display DNA barcoding trees alongside the million DNA barcode map.
As ever this is a crude first attempt, but feel free to try it and let me know how you get on.