The Apache JMeter™ application is open source software, a 100% pure Java application designed to load test functional behavior and measure performance. It was originally designed for testing Web Applications but has since expanded to other test functions.

Locust is an open source performance/load testing tool for HTTP and other protocols. Its developer-friendly approach lets you define your tests in regular Python code. Locust tests can be run from command line or using its web-based UI

The Complete Continuous Testing Platform Any test. Any device. App quality without limits.

Simplifies performance and load testing so that application delivery teams can take a standardized approach to ensure continuous performance, reliability, and scalability from development to production.

k6 Open Source is designed for load testing. Extensible to support other types of testing.

Loader.io is a free load testing service that allows you to stress test your web-apps & apis with thousands of concurrent connections.