The MTA team have released most of the source code for their San Andreas PC multiplayer addon.
Quotes copied from their site:
Formerly a closed-source project, we have migrated to open-source to encourage other developers to contribute as well as showing insight into our project’s source code and design for educational reasons.
All our source code available through this project page is licensed under the GPLv3 license. This excludes any dependencies and our net modules for both the client and server: these are still covered under our proprietary license. These modules have been excluded because of reasons involving security and cheating, but contain only a minimal amount of code.