I had thought about building a bike trailer, but I bought one from the small ads in VeloVision. It still needs quite a lot of work - see some photos. (I have changed the way the towbar attaches to the trailer since then, and the hideous shimmy it had is now fixed, or at least vastly reduced.)
More recently, I bought a single wheel trailer from eBay, very similar to Edinburgh's Revolution (no longer available), but cheaper.
But here are some links to (mainly) DIY trailers, and cargo bikes (DIY and other)
Xtracycle make an extension that bolts on the back of a standard bike, allowing cargo bags much larger than typical panniers and a "deck" that doubles as a passenger seat.
Their original FreeRadical design has now been replaced by their Leap, working on the same basic principle but fitting a wider range of bikes, with many but not all accessories interchangable.
There are complete cargo bikes compatible with the same accessories too.
Xtracycle originally published the critical dimensions of their bikes as a standard for anyone who wants to build something compatible. (They then went from freely available to password protected PDFs, and the form for requesting them seems to no longer be on their site. Copies of the old documents are (or were when I wrote this) still available on web.archive.org.)
There's some potential overlap with my tandems page, a longtail being similar to a tandem wihout a rear saddle and no rear pedals.
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