Not sure if kd-trees are lesser known, but they are neat. I only learned about them recently so to me they were "lesser known".

"In computer science, a kd-tree (short for k-dimensional tree) is a space-partitioning data structure for organizing points in a k-dimensional space. kd-trees are a useful data structure for several applications, such as searches involving a multidimensional search key (e.g. range searches and nearest neighbor searches). kd-trees are a special case of BSP trees." -- http://en.wikipedia.org/wiki/Kd-tree