Varnish
High-performance HTTP reverse proxy + cache. Poul-Henning Kamp (FreeBSD core), 2006. Written specifically for memory efficiency on modern hardware. VCL = Varnish Configuration Language for cache policy.
Used by: Wikipedia, BBC, NYT, Tumblr to offload backends. Cache hits can serve tens of thousands of RPS on a single node. Varnish Plus is commercial. ESI (Edge Side Includes) allows fragment caching of a page (cache static parts, fetch personalized parts). Modern alternatives: Cloudflare Workers + KV (edge cache), Fastly (managed Varnish-like), Nginx cache (less flexible), CDNs generally have taken over for public sites.