You do not need any special tool for load testing Node.js application. You can use any load test tools:
- ApacheBench
- Siege
- Jmeter
- Tsung
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.