I've searched a lot about stress testing a NodeJs application/module but I didn't find anything.
I have a push server application (something like socket.io) and I want to test it to ensuring there's no memory leaks and that it exhibits optimal performance.
What's the best method or tools for doing that?
You do not need any special tool for load testing Node.js application. You can use any load test tools:
From opensource and free tools I can recommend JMeter. As it has built-in Proxy Server which can record HTTP traffic. Advantage of JMeter than other tools is user friendly GUI. You can find article related Node.js load balancing and JMeter testing.