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.

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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.

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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.