Here are the 6 largest foreign exchange ECNs (in no particular order):
FXall (Accelor) + LavaFX (acquired by FXall)
FX Connect (State Street) + Currenex (acquired by State Street)
360T
Baxter-FX
ICAP (EBS)
HotSpot FX
A few links for further reading:
Euromoney FX survey 2010
London...