Bulma - Bulma is an open source CSS framework based on Flexbox and built with Sass. It's 100% responsive, fully modular, and available for free.
Strava - The #1 app for runners and cyclists
Milligram - A minimalist CSS framework
Runtastic - Runtastic offers a series of fitness apps that can be used to track your running, walking, hiking, and cycling, as well as many other fitness routines. Read more about Runtastic.
Material UI - A CSS Framework and a Set of React Components that Implement Google's Material Design
RunKeeper - Join the community of over 45 million runners who make every run amazing with Runkeeper. Track your workouts and reach your fitness goals!