PCSX2, but it needs a very good rig to run it, you need the BIOS, which you'll have to look for in places other than here, and oh yeah, any real 3d game, like KH or some of the FF games have noticeable graphical glitches/slowdowns.
I have an Intel Core Duo at 2.66GHz and I can run Itadaki Street, for the most part (one or two effects cause lag) at 100%. If that helps. I also have 2GB of RAM. Though Itadaki Street isn't especially taxing in the graphical department.
I'd highly suggest modding a cheap PS2 over emulating. Games will run better, and will infinitely be cheaper, unless you already have a somewhat good gaming rip in use.