Ask HN: What are the most innovative smartphone apps?

I'm looking for sources of inspiration for developing new apps & am wondering which existing apps do you consider to be the most innovative, in terms of exploiting the functionality that the smartphone platform offers?

Some examples:

Bump [YC 2009]: Combines location + accelerometer to match and connect bumping phones

Shopkick: Uses ultrasonic sound to confirm entry to shops and then offer rewards

Several pulse meters that are based on camera observations

etc