Quon Modular is a semi-custom prefab system from Australia. Each room is a (mostly) self-contained module, measuring 5 m x 3.1 m (16 ft x 10 ft). Buy exactly what you need placed side-by-side, stacked, or each by itself.

Room options and prices:
• Bedroom, ~$31,000
• Kitchen, ~$48,000
• Bathroom, ~$40,000
• Master Bedroom, ~$32,000
• Multi-purpose, ~$28,000

Materialicio.us loves the concept:

"For me, this is the simplest, most efficient system yet devised for a customized, prefabricated house. Design your house using their standard components, place the order, and ten weeks later it's delivered."

Few prefabs offer such a flexible approach. The weeHouse series from Alchemy Architects allows for the addition of specialized modules, such as the sleepTight, but their modules vary in size. v2world was offering a similar product in their v2shell, but last we heard, they were reworking their product line.

company: Quon Modular

(More coverage: Treehugger)