NeutrinoSX2
NeutrinoSX2 was one of the first open source PS2 emulators. It never got further than running a few purpose-built test demos.
The project’s main developer, Maud, started work on the emulator after writing code to interpret a file found on the Ico disc that contained part of the game’s source in assembly.
It is unknown why development ended, but NeutrinoSX2 left a wealth of information for anyone interested in writing a PS2 emulator, including clean, well-documented code and documentation on some of the PS2’s inner workings.
If you want to run commercial PS2 games, use PCSX2 instead.
Note: NeutrinoSX2 requires a PS2 BIOS dumped from your own console.