Here is the original concept from extra cycle. With this you can turn just about any bike (maybe not a penny-farthing) into a longtail bike.

http://www.xtracyclegallery.com/search/label/Colnago

http://rideyourbike.com/xtracycle.shtml

http://www.xtracycle.com/

 

Here is a list of manufactures offering complete builds.

http://www.yubaride.com/

 http://www.konaworld.com/bike.cfm?content=ute

Kona also offer a electric assist model.

http://www.surlybikes.com/bikes/big_dummy_complete/

http://onyacycles.com/

http://www.xtracycle.com/cargo-bicycles/radish-cargo-bike.html

http://www.trekbikes.com/us/en/bikes/gary_fisher_collection/urban_utility/tra...

http://www.madsencycles.com/

http://www.bamboosero.com/buy-a-bike.html

 

http://nankon.myweb.hinet.net/

http://www.sunbicycles.com/index.php

http://worldbike.org

http://www.fietsfabriek.nl/

 

http://www.ransbikes.com 

 Handmade Longtail frame builders

http://www.bilenky.com/Home.html

http://trafficbikes.com/

http://www.pereiracycles.com/

http://vanillabicycles.com/

http://www.sycip.com/

 http://clevercycles.com/products/stokemonkey/

 

http://www.banjobicycles.com/

http://www.edsbikes.us/

http://timetogetnaked.com/

http://www.blacksheepbikes.com/

http://www.huntercycles.com/

http://www.tomscargobikes.com/

http://www.ingliscycles.com/