When Bunnie showed up at Adafruit with the prototype for his Chibitronics Circuit Stickers we were all like "How on earth does that work?" and he was all like "Z-Axis tape - Its amazing!" And we agreed, so we decided to get some in our Materials section since its so cool.
Z-axis tape is made by 3M, and is used to bond two conductive surfaces together and allow a small (we'd say keep it under 100mA) current to flow through the tape. But here's the cool part: its only conductive across the thickness of the tape itself, not along the length or width. Mindblowing, right? We took a macro shot of the tape above so you can see how it works. Its a regular adhesive tape with little chunks of conductive material every 0.1mm or so. The adhesive does the bonding, the little chunks do the conducting.
This tape goes great with our other interesting conductive materials, such as copper tape, ITO glass or plastic, conductive inks, etc. For a quick fun project we put some tape on a Gemma and Neopixels: remove the white cover first, then stick the circuit board onto the brown paper+tape. Cut out the shape around the circuit board, then carefully peel off the brown paper. Repeat for the neopixels. Make traces on a piece of paper and stick the Gemma and Neopixels down: ta-dah, paper circuit!
Each order comes with a strip of 2" x 6" tape. Its big enough you can use it for a bunch of projects. Just cut out the size you need and save the rest.