VP trees are interesting (previously discussed here:
https://news.ycombinator.com/item?id=3304685), and there is also the MVP variant (multi vantage point trees --
https://en.wikipedia.org/wiki/MVP_tree):
Indexing Large Metric Spaces For Similarity Search Queries
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.43....