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.
CoralCDN - CoralCDN is a free and open content distribution network based around peer-to-peer technologies...
cdnjs - An open source community driven JS/CSS CDN.
CloudFlare - Cloudflare is a global network designed to make everything you connect to the Internet secure, private, fast, and reliable.
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.
Statically - The free optimization & CDN for images, CSS, JavaScript, and open source.
CDNify - Accelerate your apps and websites with the CDNify content delivery network. Superfast, simplified and affordable CDN for developers, startups and agencies.