In Apple's newest TV spot, airing during the Stanley Cup Finals, they show HealthKit being used by runners, hikers, golfers, soccer players, and other people who are in better shape than me. So Apple has started their promotion of HealthKit even though they won't actually tell us how it works yet. If anything, the commercial added more questions than it answered.
So far, we know that HealthKit will collect fitness and health data, as well as be able to receive data directly from your physician. We also know that Apple is working with Nike and the Mayo Clinic on the project. But as far as fitness data collection goes, we don't really have a clue what Apple is planning. The iWatch is a maybe, a Fuel-Band type product could work considering the Nike partnership, but smart earbuds seem unlikely.
But in the commercial, we see a variety of other devices collecting data, some wearable and some not. A swimmer has a small sensor to determine speed; a golfer wears a sensor on their wrist to judge the swing; a man stands on a smart-scale feeling defeated because HealthKit says he hasn't lost a pound. All of these products actually already exist, and aren't made exclusively by Apple. My trained tech eye spotted the Zepp Golf Sensor, Misfit Shine, the Withings Scale, miCoach, Wahoo, Nike+, and Argus. (Post in the comments with any I missed.)