A choker (tight necklace) which measures your heartbeat (pulse) and shows it to others via a set of LEDs. It may also optionally communicate the same info to the wearer via a vibrator. Although theoretically heart rate should be known via senses intrinsic to humans, in practice it is actually quite difficult to know, and I think there are many interesting social interactions that pulse-display could trigger.
Currently Eric Boyd is working on this device.
Sensor: I think the easiest way to sense the heartbeat would be via a microphone. An alternate method involving sensing of movement would probably require the choker to be too tight to be comfortable. The device should also be able to sense actual sounds, like music, and optionally use that instead of heartbeat as the signal to be displayed. I am also curious if the device would sense your breathing as well.
Display: Initially, only to people other than the wearer, via a string of LEDs inside the choker. I imagine the pulsing centered on the LED in the center, and going out to all, then coming back in, similar to how some star trek (?) lights worked when the computer talked. There may also be a lone vibrator, which could be used to signal to the wearer a variety of things (like pulse, but also time or low battery etc). Although this is really stretching the definition of "display", the device might also log data for later retrieval via USB.
Armature: Choker, plus it is likely that it will have to have a pendant hanging from it, which would contain the circuit and battery. The vibrator might also be in the pendant.
Electronics: I am very tempted to attempt this project as an entirely analog device. This is partly because even if I did digitize the sound signal, I would have no idea how to properly handle the data from there to actually extract heartbeat info. I'm hoping that an analog filter could simply have a digital output - high for the fraction of a second when your heart makes the most noise. Pump that through a few timer chips and presto.
I already have a set of chokers with surface mount LEDs inside (I've shown this at prior meetings during show and tell). I am unsure of how the LEDs are wired - it's possible that they are all controlled from the same wire, in which case the device will not be able to display as above, and instead will simply flash. I got the chokers from Rebeccas for only $2.15 each, so if they are suitable they make great kit components.
For handling the sound data, I anticipate that I'll need some op-amps. Noisebridge may already have suitable parts for prototyping.
For inspecting signals to learn how the microphone actually works (what it hears) I will likely use an o-scope.
I've done some google searches, but I haven't been able to find anything like this. There are some pulse-sensing watches, for instance Chinavasion: Exercise Watch - pulse + calorie reader or see some reviews at Heart Rate Monitors. These devices almost universally have a small display, but I think a haptic solution could be greatly superior.
On a small tangent, I also think the Truth Wristband, which measures galvanic skin response (like a lie detector) has a similar kind of social implication...