https://github.com/ValveSoftware/source-sdk-2013
It is not that the quality of the code is high, but just that it is well organized, and everything seems like it was written by a beginner. That makes it wonderfully easy to read and follow the logic.
Since have played around with the Source Engine, I follow the KISS principle with coding with high priority. Rarely trying to be clever, or try to over-do abstractions.
[1] https://github.com/ValveSoftware/source-sdk-2013 (originally released in 2004)
The games now build and run clients on Windows, OSX, and Linux. Dedicated servers are supported on Windows and Linux.
Steam Pipe (the new Steam content delivery system) is supported by the sample mods. Existing mods can change their gameinfo.txt to match the new format and gain Steam Pipe support.
Support for Virtual Reality via the Oculus Rift has been added to the SDK. Running a compatible mod with -vr on the command line will run the mod in stereo and enable head tracking on the Rift."
Sourcecode: https://github.com/ValveSoftware/source-sdk-2013
Installation guide: https://developer.valvesoftware.com/wiki/SDK_Installation