The core team is two guys, but they hired/contracted additional people to help with QA/porting:

    - Allan Blomquist (Wii port)
    - Paul Hubans (QA and Production associate)
    - Maks Verver (Linux port)

http://goofans.com/world-of-goo/about-2d-boy

http://www.mobygames.com/game/windows/world-of-goo/credits