H2O is described as 'An optimized HTTP server / library implementation that can be used either as a standalone server or a library. Supported Protocols: HTTP/1.0 (http and https) HTTP/1.1 (http and https) Websocket (RFC6455, both ws and wss)' and is an app in the Network & Admin category. There are seven alternatives to H2O for a variety of platforms, including Linux, BSD, Mac, Windows and FreeBSD. The best alternative is nginx, which is both free and Open Source. Other great apps like H2O are Caddy (Free Personal, Open Source), Cherokee (Free, Open Source), gatling (Free, Open Source) and TrustLeap G-WAN Web Server (Free).
nginx [engine x] is a HTTP and reverse proxy server, as well as a mail proxy server written by Igor Sysoev. It has been running for more than five years on many heavily loaded Russian sites including Rambler (RamblerMedia.com).