There seems to be a few proxies that can convert a WebSockets connection to TCP, however I couldn't find anything that goes other way around. Having the task of sending crafted messages to a WebSockets connection made me write one. Having a TCP to WebSockets proxy allows me to use netcat as a WebSockets client.
I have used the code developed by our Quentin, and added stuff like proper command-line options, upstream proxy support (to tunnel it through Burp) and making it multithreaded.
The tool is written in python and uses the websocket library.