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.)
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. They do complete cargo bikes compatible with the same accessories now (2013) too.
Xtracycle published the critical dimensions of their bikes as a standard for anyone who wants to build something compatible. They went from freely available to password protected PDFs, and are now available via e-mail from a form if you agree to Xtracycle's terms and conditions.
Copies of the old documents are (or were when I wrote this) still available on web.archive.org.
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