- R-trees, as in R-TREES: A DYNAMIC INDEX STRUCTURE FOR SPATIAL SEARCHING, Antonin Guttman, Proc. ACM SIGMOD Int. Conf. on Management of Data, 1984.
- Greene-trees, as in An Implementation and Performance Analysis of Spatial Data Access Methods, Diane Greene, Proc. 5th IEEE Int. Conf. on Data Engineering, 1989.
- R*-trees, as in The R*-tree: An Efficient and Robust Access Method for Points and Rectangles, Beckmann, Kriegel, Schneider and Seeger, Proc. ACM Int. Conf. on Management of Data, 1990
- X-trees, as in The X-tree: An Index Structure for High-Dimensional Data, Berchtold, Keim and Kriegel, Proc. 22th Int. Conf. on Very Large Databases, 1996
Future work planned includes performance enhancements, incorporation of more index structures, and some work on supporting more optimal indexing when the entire set of data is known at index creation time; for more details, see the TODO file in the binary distribution.
You can get this via Quicklisp.
As per #lisp discussion, the new official source for this is the github repository, which is a conversion of the darcs repository.