Stillwater Dwellings offers a cool "mix and match" approach with their interchangeable series:
It’s as easy as A-B-C: start with your favorite Living Module (A.). Add a Connector Module (B), then complete your design by attaching your desired Bedroom Module (C). Its as easy as that! Each module is designed with a universal connector location so they all fit together. You can arrange them in any A-B-C combination you want.
The company will complete its first home in June 2009 in Bend, Oregon.  Some background from their site:
We founded Stillwater Dwellings on one primary principle: Quality, prefabricated homes do not need to be expensive. 
...
All of our homes are built off-site, in state-of-the-art manufacturing facilities. These facilities are climate controlled, and use a consistent, high-caliber workforce of carpenters and tradesmen.
...
From first contact with us to final cleaning, your home can be "move in" ready in about 3-4 months
They also have a set of fixed models:
   
        | Model | 
        Price | 
        Size | 
        BR | 
        Bath | 
        Stories | 
   
   
  | 
   
         miniONE | 
        $49,950-$89,000 | 
        370 sf | 
        studio | 
        1 | 
        1 | 
   
   
         miniTWO | 
        $79,500-$117,000 | 
        585 sf | 
        1 | 
        1 | 
        1 | 
   
   
  | 
   
         sd111 | 
        $113,100-$169,650 | 
        870 sf | 
        1 | 
        1 | 
        1 | 
   
   
         sd112 | 
        $117,000-$175,500 | 
        900 sf | 
        2 | 
        1 | 
        1 | 
   
   
         sd121 | 
        $130,000-$195,000 | 
        1,000 sf | 
        2 | 
        1 | 
        1 | 
   
   
         sd122 | 
        $162,500-$243,750 | 
        1,250 sf | 
        3 | 
        2 | 
        1 | 
   
   
         sd123 | 
        $199,950-$302,250 | 
        1,550 sf | 
        3 | 
        2 | 
        1 | 
   
   
         sd131 | 
        $214,500-$321,750 | 
        1,650 sf | 
        2 | 
        3 | 
        1 | 
   
   
         sd132 | 
        $260,000-$390,000 | 
        2,000 sf | 
        3 | 
        3 | 
        1 | 
   
   
         sd133 | 
        $299,950-$441,750 | 
        2,325 sf | 
        3 | 
        3 | 
        1 | 
   
   
  | 
   
         sd211 | 
        $253,500-$380,250 | 
        1,950 sf | 
        3 | 
        3 | 
        2 | 
   
   
         sd212 | 
        $227,500-$332,500 | 
        1,750 sf | 
        4 | 
        3 | 
        2 | 
   
   
         sd221 | 
        $344,500-$503,500 | 
        2,650 sf | 
        3 | 
        3 | 
        2 | 
   
   
         sd222 | 
        $390,000-$570,000 | 
        2,990 sf | 
        3 | 
        3 | 
        2 | 
   
   
         sd231 | 
        $383,000-$560,500 | 
        2,950 sf | 
        3 | 
        3 | 
        2 | 
   
   
  | 
   
Hat tip: Jetson Green on March 25, 2009.