k6 is the name of a reliability testing tool originally created by a company with the same name, but now part of Grafana Labs. I worked at k6 from January 2020 to October 2021 as the Head of Developer Relations.
The tool itself is mainly built for performance and reliability testing, and is written in Go. It is open-source and free to use by anyone. In addition to the open-source offering, the company k6 also offers a fully managed cloud service called k6 Cloud.
One of the main things that differentiates the tool from others in the same category is the fact that it’s blazingly fast. Being written in go, a compiled language created with concurrency in mind, it is able to churn out high volumes of concurrent virtual users on modest hardware.
Another differentiating quality that I personally really like is that it is code-based, while most competing tools either are configured in a declarative way, using for instance YAML, or GUI-based.