I just did some research on QuadTrees, because I am thinking about using them for an efficient in-game event system, that would be suitable for big MMOs to be able to process the vast amount of events on the server-side.
The most interesting advanced QuadTree implementation I came across is a so called Skip-QuadTree, which has several advantages, but do the reading yourself if you are interested:
#Skip-QuadTree publication
#Presentation of the same group
#Another paper dealing with quadtree and more
#Presentation of quadtree utilized in distribution system
