Name: AppBuilder 5
Rating: 4.9 (70 reviews)
Author: flowstate

Build your own datafield! No coding required.

The latest version of AppBuilder allows you to create your own calculated data field based on a simple math formula, like "Cadence / 2".  Also comes with conditional logic, alerts (beep and vibes), text display, functions (e.g. lapavg, avg, min, max, log). and the ability to record data to the FIT file so you can see the graph in Garmin Connect. 

You can store up to four formulas in the app and switch between them without a phone or PC.

Full documentation and examples: https://ciq-appbuilder.blogspot.com/p/examples.html
Discussion and updates: https://forums.garmin.com/developer/connect-iq/f/showcase/5428/data-field-appbuilder
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AppBuilder5 is one of four clones, so you can have 2-4 formulas per activity (depending on device) and store up to 16 formulas. All my apps, including clones:
https://apps.garmin.com/en-US/developer/8545c609-c030-4c0c-b3d8-4bc63ebdc901/apps?sort=name
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Some examples of things you can build with AppBuilder:
• Speed/pace, 5 second average
• Power, 5 second average
• Normalized Power, Intensity Factor, FTP
• Yacht race timer, marathon predictor, distance to next milestone/aid station (with beep/vibe alerts)
• Elevation grade, elevation change, lap ascent/descent, lap elevation change
• Repeating countdown, Interval rest/work timer
• Stride length
• Count sprints, time spent sprinting
• Last lap average heart rate (HR)
• Last lap maximum heart rate 
• Distance/speed in opposite units
• Average cadence, ignoring zeros
• Display multiple values at the same time (e.g. Current HR/Average HR).
• Alternate between multiple values every 5 seconds 
• Time since last pause
• Total time paused
