The effects were mainly created by Rhythm and Hues Studios, an international company based in California but with offices in India and elsewhere. The effects for this film were largely made from branches in India, Canada and Taiwan; a number of artists worked together using cloud computing technology to collaborate.
CG animation was used to create the tiger in Pi and other effects. The studio spent over a year working on R and D alone, to perfect their use of CG.
Other studios, including Crazy Horse Effects, were also involved, but Rhythm and Hues' contribution was by far the most important. The movie won an Oscar for visual effects in 2012 - one of six awards including best director (Ang Lee) and best picture - and is the 5th highest performing movie at the US box office this year.
The man in charge of the visual effects was Bill Wesenhofer - you can read an interview with him here.