Grafana k6 is an open-source, developer-friendly, and extensible load testing tool. k6 allows you to prevent performance issues and proactively improve reliability.

Mockoon is the easiest and quickest way to design and run mock APIs. No remote deployment, no account required, free and open-source.

FillDB is a free tool that lets you quickly generate large volumes of custom data in MySql format to use in testing software and populating databases with random data.