GPT-3 Playground is the web interface for OpenAI's GPT-3 API. It is likely the original prompt engineering tool that most people have used. As such, it is extremely well known.
GPT-3 Playground provides a clean interface with a few different dials for modifying GPT-3's behavior. It also provides access to different GPT models of varying parameter sizes and prices. They provide free credits, which expire after 3 months.