websocketd - Turn any program that uses STDIN/STDOUT into a WebSocket server. Like inetd, but for WebSockets.
µWebSockets - Simple, secure & standards compliant web I/O for the most demanding of applications.
µSockets - Cross-platform async networking and eventing foundation library written in standard C.
Sockette - Cutest little WebSocket wrapper.
websocat - Command-line client for WebSockets, like netcat (or curl) for ws:// with advanced socat-like functions.