The source code for Darwin 6.5 - the heart of Mac OS X - is available for download.
The update includes a large number of source projects, reflecting the vast quantity of work conducted by Apple and its developers in making OS X 10.2.5, which Darwin 6.5 corresponds to, Apple said.
Apple said: "Darwin 6.5 is what we call an 'on-cycle' source-code release, where the corresponding Darwin source is made available soon after a new release of Mac OS X to customers.
"Several projects such as CUPS and Rendezvous will continue to do 'off-cycle' releases, where the source code is updated more frequently than our commercial releases."
Darwin 6.5 is available for download from Apple's developers site, where individual packages are also listed.