MQTT Load Simulator
There are various MQTT stress testers out there, but they often give an inaccurate representation of server performance, because they don’t factor in their own performance limitations. It’s really hard to see when it’s the stress tester that is being overloaded, or the server. MqttLoadSimulator aims to allow better control of traffic generated and show its own performance:

Since version 1.2.0 it’s much faster and distributes load more evenly in time.
The code is on Github, but you can also download a binary here (AppImage is preferred).