Masimo makes medical-grade pulse oximeters, which measure the amount of oxygen in your blood. Now available (for US$249) is the iSpO2, a more consumer-grade model for athletes and aviators who need to track their O2 levels but aren't using it for a medical need.
Like the Tinke, which we covered last year, the iSpO2 will track your O2 levels over time via a companion iOS app. Unlike the Tinke, the app is actually quite good. The hardware is also outstanding, with Masimo taking its expertise in medical gear and making a less expensive version for consumers. You just clip one end to your finger (which you've no doubt seen in hospitals or TV shows about hospitals), then attach it to an iOS device and use the app to start tracking.
Another advantage over Tinke is this provides continuous monitoring, key for athletes needing to see how their oxygenation levels are doing over time.
Engadget has a gallery and hands-on if you're interested in seeing it in action.