Killer app? I don't know about killer, but here are some super useful things I can imagine smartwatches doing:...Basically, most of the potential with this device is doing things which phones can already do but in a less intrusive manner.

We shouldn't ever dismiss this avenue for product/business development: Duplicating an existing functionality/service but with greater convenience. One of the charges raised against both the iPhone and iPad, was that they merely duplicated functionality that was already in other devices. However, making that functionality more convenient and mobile turned out to be of incredible value to the market.