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.
CloudFlare - Cloudflare is a global network designed to make everything you connect to the Internet secure, private, fast, and reliable.
cdnjs - An open source community driven JS/CSS CDN.
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CDN77 - Content Delivery Network - website speed acceleration with CDN77. 28+ PoPs, Pay-as-you-go prices, no commitments.
Statically - The free optimization & CDN for images, CSS, JavaScript, and 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.