The ndb (MySQL Cluster) source code.
MySQL's shared-nothing in-memory feature is practically a DBMS by itself. We generally use the term "ndb" when referring to the storage engine, and the term "MySQL Cluster" when referring to the combination of the storage engine and the rest of the MySQL facilities.
The sub-directories within ndb are:
bin --- Two script files
config --- Files needed for building
demos --- Demonstration scripts
docs --- A doxygen output and a .txt file
home --- Some scripts and .pl files
include --- The .h files
lib --- empty
ndbapi-examples --- Examples for the API
src --- The .cpp files
test --- Files for testing
tools --- Programs for testing select, drop, and so on