Open source caching software is the engine the powers many CDNs. Without it, the CDN industry would most likely be smaller today. Free open source software enables startups to start small, and scale later. Fastly uses Varnish, Netflix & CloudFlare use Nginx, and Comcast uses Apache Trafficserver. More established CDNs like Akamai and Limelight, probably have their own caching platforms, where every line of code has been written from the ground up, customized to support their environment.
The different types of caching software are best for certain types of content. Netflix tested several caching software platforms, and decided to go with Nginx. Fastly decided on Varnish because it supports high currency (simultaneous connections). Caching software is one of the many pieces of software required to operate a CDN. It’s not like you can just load caching software on severs across different data centers, and expect to be a CDN. Applications need to be developed for reporting, control panels, web services API, and much more.