PS2emu is an extremely basic PS2 Emulator. It was in development by a trio of talented emulator devs, namely Roor who made AndriPSX, and also shunt and scar_T. PS2emu never really gained enough development time to actually become anything to great.
It ended up as an attempt to trial at several new techniques that would enable the PS2 to be emulated much faster then other emulators at the time, however the Dynamic Recompiler featured in the only alpha they released was only about 10% complete. It was a good proof of concept, but sadly as said before, development never really gained much momentum and it never saw another release.
There was signs of Roor coming back to finish off his development on the emulator, but he seems to of disappeared again as well.