PCSX2 is a PlayStation 2 emulator for Windows, Linux & Mac, started by the same team that brought you PCSX (a Sony PlayStation 1 emulator).
PCSX2 vs Play! PS2 Emulator opinions
it has a lot of options that can make games run smoothly and is lightweight.
Open Emu is an open source project to bring game emulation to OS X as a first class citizen, leveraging modern OS X technologies such as Cocoa, Core Animation and Quartz, and 3rd party libraries like Sparkle for auto-updating.
OpenEmu vs Play! PS2 Emulator opinions
Its not an emulator for PS2