Cloudflare

Cloudflare sits in front of a website as a reverse proxy, providing CDN caching, DNS, a web application firewall and DDoS protection.

That position is what makes it useful: traffic reaches Cloudflare before your server, so attacks are absorbed at the edge and cached content is served without touching your origin at all.

Its free tier covers CDN, DNS and basic DDoS protection, which is unusual generosity and the reason it fronts a large share of the web. Be aware that proxying means Cloudflare terminates TLS, so it sees traffic in cleartext.