Inspired partly by the image viewers mentioned earlier, and tools like Google Finance's plot of stock prices, I've built yet another demo of one way to view large trees.
You can view the demo here. On the left is a thumbnail of the tree, on the right is the tree displayed "full scale", that is, you can read the labels of every leaf. In the middle appears a subset of any internal node labels. Top right is a text box in which you can search for a taxon in the tree.
You can navigate by dragging the scroll bar on the left, dragging the big tree, or using the mouse wheel (and you can jump to a taxon by name). It has been "tested" in Safari and Firefox on a Mac, I doubt it works on Internet Explorer. Getting that to happen is a whole other project.