Electro-Luminescent wire.
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I am a little late to the game on this stuff -- apparently hobbyists have been using it for crafting. For example, who wouldn't want a Tron outfit?
To date, I have mostly sheltered readers from the geekiest of my hobbies: model railroading (Yeah, I know what you are saying -- how can anyone who spends hours a day at a computer writing on arcane bits of business and economics issues possibly be anything but cool?) This may soon change, as I am starting a new N-scale layout and I will probably inflict some in-progress photos on you folks. To get an idea just how crazy I am, I build my own track from wood strips and bundles of rail and tiny, tiny spikes -- so we are not just talking about putting the old Lionel out on a green table cloth.
Anyway, for some time I have wanted to build a layout that is primarily meant to be run in the dark as a night scene. So I am experimenting with a lot of technologies, from florescent paint to tiny LED's to small bulbs to get ideas for various scenes. The EL wire turns out to be a dead ringer for scaled down neon, so I expect to use a lot in the city part of the layout.
I will leave you with a photo of the layout that probably inspired more people (including myself) into the hobby than any other - by the master, John Allen:
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If you get intrigued with his work, more photos are here.
I wish I had more pictures of my old work, but they seem to have been lost in a move. All I have left is a few poor-quality, poorly-scanned under-construction photos of my first layout from years and years ago.
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Postscript: Can a hobby be geeky if Rod Stewart shares it? He has built an absolutely stunning layout - one photo below and more here. 
And yes, the work really is his own, he didn't just pay someone to build it for him.