Being a sucker for punishment I keep returning to the problem of visualising big trees. My latest quick and dirty demo uses Google Maps and generates image tiles using spatial queries on a MySQL database. Blog post here http://iphylo.blogspot.co.uk/2015/09/visualising-big-phylogenies-yet-again.html and live demo here http://bionames.org/~rpage/deep-tree-experiments/index.html
There are two algorithmic/design issues to solve for this to be really usable. The first is reducing the number of nodes drawn at each zoom level (ideally to something roughly constant per 256 x 256 pixel tile), the second is determining what labels should be displayed at what zoom level. I’d welcome thoughts on that, I recall some work has been done on the first problem as part of the TreeJuxtaposer project.