Tiny, easily embeddable HTTP server in Java.
Eclipse Jetty - Web Container & Clients - supports HTTP/2, HTTP/1.1, HTTP/1.0, websocket, servlets, and more
http-kit is a minimalist, event-driven, high-performance Clojure HTTP server/client library with WebSocket and asynchronous support
Rapidoid - Extremely Fast, Simple and Powerful Java Web Framework and HTTP Server!
A Java event based WebSocket and HTTP server
A Java WebSocket/HTTP server based on the Atmosphere and Netty Framework
Java tool for mocking HTTP server behaviours for testing HTTP client code