Loader.io is described as 'simple cloud-based load testing service'. There are more than 25 alternatives to Loader.io, not only websites but also apps for a variety of platforms, including Windows, Linux, Mac and Self-Hosted solutions. The best alternative is Apache JMeter, which is both free and Open Source. Other great sites and apps similar to Loader.io are locust (Free, Open Source), Yandex.Tank (Free, Open Source), Gatling (Free, Open Source) and Loadrunner (Paid).
Apache JMeter is an open source Java application, designed to load test functional behavior and measure performance. Originally designed for testing Web Applications, it has since expanded to other test functions as well.
OctoPerf is a SaaS load testing solution, based on JMeter. Stress test your web and mobile apps within minutes. Design realistic virtual users. Scales to thousands of user from multiple locations simultaneously. We take care of the infrastructure.