The previously private massive mod for Counter Strike 1.6 is now open source. Mod built on AMXMODX.
At the heart of the project is an extensible core and plugin system, allowing modifications via well-defined interfaces (sorry, no docs - you'll have to dig around the SMA files).
Note: This mod comes without any support or future development. You can find documentation in the source code, but I don't have the time to whip up install/use/extend docs.