Netplay

GGPO

Arcade emulator built around netplay.

RetroArch

...Someone should really try this out already.

RA uses GGPO netcode, it should be lag-free if everyone involved emulates at fullspeed, don't live on the opposite side of the world from eachother and everyone has decent internet speeds.

Dolphin

Official Netplay Guide

Note: The devs are working on integrating Nintendo Wifi Connection to connect emulators to Nintendo's own servers.

PCSX2

UnOfficial Netplay Build

Netplay has never officially been added to PCSX2, but someone tried making their own branch a few years ago, only about 7 fighting games work and the project has been abandoned.

Note: You can connect to Sony's own servers though: How To Play PCSX2 Online (See Online PS2 Games for what games still have servers up)

If you don't have a very powerful CPU you may want to not even bother.