A camera that takes a very thin photograph - and then another - and then places those photos next to each other. So you are looking at the same place just later in time. So seeing lots of time piled on top of itself. However you talk about it is hard to get your head around but it makes for beautiful work.
Here are three artists working with slit-scan across video, photography and code.
Jay Mark Johnson
Makes classic slit camera photographs. http://goo.gl/MqYhk
Daniel Crooks makes insanely beautiful videos:
to slitscan iphone apps:
Best Comment: "So THAT'S what the world would look like if c was 70 mph..."
There is a very thorough and excellent overview of the form here by Golan Levin: http://www.flong.com/texts/lists/slit_scan/