There are currently 5 big vfx houses.
WETA, ILM, RYTHM & HUES, SONY IMAGE WORKS & DIGITAL DOMAIN.
there are a lot of other awesome fx houses like Hydraulx, hybride, Imagine Engine and bunch more that can also deliver fx as good the big 5 but they are not owned by huge people so they don't get the respect they deserve, which is wrong. Fans need to stop whining when those big 5 aren't working on those films. Sometimes those big 5 fx houses find things very challenging and hence drop out of the project.
Weta = Peter jackson
ILM = george lucas
Rythm & Hues has managed to make some name for themselves eventhough it is not owned by someone who is huge in hollywood
SIW = owned by sony which is big
Digital Domain = Michael Bay owns it now