Micro movement sensing
Use the orientation sensor in a cell phone to monitor small regular movements. For example, the movement due to the heart beat. It might be possible to measure breathing movement. It may also be possible to measure anomalous movements–tremors, the sway due to microadjustments involved in standing.
Also, if the heartbeat moves a phone to a noticeable degree, does this make cell phone photos blurrier? If so, add heartbeat detection to the camera app–have pictures be snapped between heartbeats.
Basically, orientation data would be collected, and a frequency analysis done to detect the freq and amplitude of the movements. I don’t know how fast the orientation can be polled. If it is too slow, it may be possible to use intermittent polling at precise times to identify the frequency of movements.
A related use would be as a shadow boxing app. This would clearly be better as a wrist strap standalone device, but might work as an app. Hold the phone in a hand (or strap it firmly to the wrist), and follow the movements of the hand/arm, recording punches and the speed of them. The user would indicate the hand being scored in setup, and then as hands are switched, the punching of the two could be compared. Groups of friends could play together to see who can punch the fastest or do the most punches in 30s.
This app would run simultaneously on two phones. One phone would display bands of pure red, blue, and green. A sample would be placed across one half of the bands. The second phone would take a picture of the first phone. Image analysis would compare the brightness of the control and sample covered regions and calculate absorbance in the three channels. Cell phone displays are either OLED or IMOD. There are a range of displays used in phones, so this would never be super accurate without calibration. The OLED displays seem to have fairly narrow spectrum pure colors while the iphones have broader colors.
This could be used either as an exploration tool–test substances and record spectra, or reference data could be used to make guesses at substances.
Or you could use a diffraction gratings and make a real spec.