SRB2 is the start of where I consider myself a 'serious' programmer, rather than a programmer-to-be still learning the ropes. I was brought onto the project in 2009, after I had made some modifications of my own for the game. Due to the extensive amount of time I've spent working on SRB2, there is almost nothing in the game that I haven't touched in some form; this includes physics programming, enemy and object AI, rendering, level design, and even to a minor extent graphic design.
If I had to pick one single thing I feel most proud of, however, it would be introducing a one-button GIF saving option which allows anyone to start a GIF recording at any time; taking advantage of the fact that the game still uses a 256-color palette as a stylistic choice. The GIFs that this feature saves are automatically optimized for being uploaded to the web to a service like imgur or gfycat. To say the least, being able to see concepts in motion as they're implemented (instead of just static screenshots) greatly helped us as a development team, and having members of the public be able to submit them along with bug reports also assisted debugging.
As a free, open-source project, I have submitted multiple accepted patches to the repository; and what doesn't get accepted to the main repository usually goes into one of the closely related forks. The source code for the project as a whole is available on GitHub. For specific change sets, there's the Monologuing fork for automatically handling extended length messages, and the Direct Messages timeline (which is featured on the glitch-social fork).
Because of the modular nature of this bot, the source code is split into multiple repositories. The source for the main IRC bot as well as all the modules I had programmed myself, the channel moderation module and the Fortune game module, are all available on GitHub.
The source code for SignPrintf can be found on GitHub.