Zoo : Multi-System Emulator
Zoo is the longest-running and deepest/most complicated collection of software I've ever worked on, including my actual work as a professional software developer.
Developed by uchinokitsune@ and myself, we originally began by designing a Sega Dreamcast Emulator called "Penguin", but it slowly morphed into writing our own JIT compiler ("fox"), writing multiple processor cores, graphics and processor debugging tools.. It eventually grew to include a very basic shell of a PS1 emulator (It can run through a few games completely).