Eclipse Jetty - Web Container & Clients - supports HTTP/2, HTTP/1.1, HTTP/1.0, websocket, servlets, and more
Tiny, easily embeddable HTTP server in Java.
Rapidoid - Extremely Fast, Simple and Powerful Java Web Framework and HTTP Server!
http-kit is a minimalist, event-driven, high-performance Clojure HTTP server/client library with WebSocket and asynchronous support
A Java event based WebSocket and HTTP server
A Java WebSocket/HTTP server based on the Atmosphere and Netty Framework
This is the simplest fastest full fledged web server we could come up with.