jsDelivr's answer
jsDelivr is the only free CDN that aims to be production ready even for the most critical workloads. It has multiple failover systems for all of its components to ensure 100% uptime and uses multiple CDNs and datacenters for better performance.
cdnjs - An open source community driven JS/CSS CDN.
Google Open Source - All of Googles open source projects under a single umbrella
CloudFlare - Cloudflare is a global network designed to make everything you connect to the Internet secure, private, fast, and reliable.
LaunchKit - Open Source - A popular suite of developer tools, now 100% open source.
KeyCDN - KeyCDN is a high-performance Content Delivery Network (CDN). Lowest price globally at $0.04/GB with HTTP/2 Support and free Origin Shield.
Open Source @IFTTT - A collection of IFTTT OSS projects.