NeutrinoSX2
NeutrinoSX2 was one of the first open source PS2 emulators. The project ultimately never ended up getting further then being able to emulate some purpose built functionality test demos for the PS2.
The projects primary developer was Maud, who got into developing the emulator after writing code that interpreted a file that he found in Ico that had source of part of the game in assembly.
Its unknown why development ended, but NeutrinoSX2 left a wealth of information for those who were interested in developing an emulator for the PS2, it featured clean and well documented code as long as documentation of some of the inner working of the PS2.
If you are after an emulator that can actually successfully run commercial PS2 games, then we highly recommend you use PCSX2.
Note: NeutrinoSX2 requires the PS2 Bios files to be able to run.