HiJack is an awesome project.  For parts that cost $2.34 in 10k volumes, students at the University of Michigan have created a device that can connect external sensors to a mobile phone through the audio jack.

Sensing. We envision a range of sensorboards including ozone, carbon monoxide, DVM, blood pressure, blood glucose, and others. But today, we only have four daughterboards: (1) a simple demo board with temperature/humidity sensors, PIR motion sensor, and potentiometer used on the early HiJack prototypes; (2) a 3-lead EKG sensor; (3) a basic soil moisture sensor; (4) a breakout board for fast prototyping on the latest generation of HiJacks.

I can’t imagine costs would be that much higher to make a dozen of these.  It will be a platform for people to create novel technologies (hence more jobs, individual tech empowerment)- all sorts of sensors and hardware mobile attachments, extending the range and applications for mobile phones.  It would be exciting to see this extended for phones widespread internationally, like the most popular phone in the world, and so hardware mobile accessories could be extended to a far wider population.  Imagine health sensing, chemical sensing, air quality sensing, all with the phone.  This could be an additional tool for doctors in the field- powering all sorts of useful, inexpensive medical hardware.

All of the source code and schematics are available for free.  Now to brainstorm, what awesome stuff would you want to power with your phone?  What could you make that would be most useful?