Part of the data science revolution is the adoption of sensors to capture data that can inform the analytics.  Our solution is only as good as the underlying data so we’re big advocates of data collection across the endurance sports ecosystem.  Sensors and apps focused on your bio markers, body composition, power, heart rate, sleep, diet, hydration, etc all play a meaningful role in unlocking what makes you successful in endurance sports.  This list below contains the devices and sensors we advocate with details as to our level of support.

Apple Health
Health data ecosystem app allowing developers to read/write health related data users have authorized to be shared. Activity, Mindfulness, Nutrition, Sleep and Bio Markers. Majority of health related apps and sensors leverage this platform and is adopting it as our standard like Garmin and Strava have already done.

Currently many users have their weight automatically updated in Strava via Apple Health. Doing so allows you to see the impact weight changes have on your threshold factors.
Garmin Connect
Garmin Connect
Mobile app supporting your Garmin Connect account. Supports Apple Health out of the box and their fitness trackers and various other sensors leverage this app. This app is key for getting your workouts automatically uploaded directly to Garmin Connect, Strava and Training Peaks.

Our intent is to establish an interface with Garmin Connect as we'd like to have the source FIT file for every activity. These files contain a wealth of data not available through the Strava API.
Training Peaks
Leading endurance athlete workout log and coaching platform. A successful business for nearly twenty years, TP is THE coaching platform for the endurance sports world. Advantage is several proven fitness variables for people that train with power and purpose. They also curate excellent fitness and coaching related content.
Indoor Cycling outdone! Great tool for flexibility on when to fit rides into your busy schedule and excellent way to get structure with your training.
Body Cardio by Nokia (aka Withings)
IoT scale that measures common body composition metrics like weight, body fat%, hydration, BMI and some interesting heart health metrics like pulse wave velocity.

Withings makes a wide cadre of internet enabled devices. I personally use both the Body Composition and their Blood Pressure Monitor. You step on the scale and it sends your data to the cloud in minutes giving you a nice history and allowing apps that leverage Apple Health to learn of weight changes.
MightSat by Masimo
Mobile fingertip pulse oximeter. Works via bluetooth connection to iPhone. Medical grade device from reputable organization. Device is already Apple Health compatible and has interfaces to Training Peaks.

Our plan is to support this device through the Apple Health interface once we have our iPhone app up and running.
Garmin Edge 520
Bike bike computer from Garmin. Good balance of data and features for a reasonable price. Supports auto Strava/TP/Garmin Connect upload via Garmin Connect on your mobile.
Crank based power meter under the SRAM umbrella.
powertap G3 HubRear wheel based power meter One of least expensive ways to add Power to your bike.
I believe this may have been the first power meter but I'm not 100% sure of that!
VariousAll of the devices listed on this wikipedia page are supported by Strava and Garmin.

DC Rainmaker has a guide to Power Meters here.

This is a great book on training and racing with a Power Meter.
Leading power meter for runners. Power is the direct measure of your work. Using power, Stryd monitors your running intensity, mechanics, and efficiency, quantifies your running capabilities, guides you to train purposefully, run efficiently, and race faster. Stryd has a very nice website that includes some sophisticated charts and metrics not yet supported within Strava.