A battery powered Tesla coil, in the shape of a rifle. Build details here: http://hackerfriendly.com/the-tesla-gun/
Sparks are quite tricky to film. There is a beautiful purple-blue corona field that can barely be seen in the vid, even with this camera (a Nikon D7000).
The maximum arc draw on this run was about 12" or so.
Channel: Sci & Tech Channel