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 you everyone involved emulates at fullspeed, don't live on the opposite side of the world from eachother and everyone has decent internet speeds.
Dolphin
- If you don't have a very powerful CPU you may want to not even bother.