I am the author of AlchemyDB: http://code.google.com/p/alchemydatabase/
I would very much like to add geospatial indexing in the form of R-trees (most likely) and need some advice on whether or not there any C/C++ packages that have simple stripped down R-trees. I am a big fan of small codebases that are easy to read.
If anyone has any suggestions or knows of any experts that I could bug (I can pay for their time), I would be grateful