Weta, of course. The Warthog above, as seen on Pixelated Geek, was made by a company called Custom Creations, and was built for the Halo fan film, Operation Chastity. That's right, it was made for a fan film. Not to put too fine a point on it, but had we been tasked with building a Warthog, it probably wouldn't even be as good as modified Power Wheels. Suffice it to say, it's pretty nice.
To see some pics of the Warthog's construction process -- it took approximately a year to build -- head over to the official Operation Chastity website. The project begins filming in 2010.