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Arena is a swiss army knife for everything. There is a command window and there is an editor. You can use both of these to compose and fiddle with data and payloads of various nature. It is simple, elegant and extremely powerful in the hands of experienced users.
Arena provides source code highlighting, contextual help and visual aid to guide you through the process of whatever you do. You can even use Websecurify's browser API to send and receive HTTP requests (like Resend and Rest) and observe browser traffic (like HTTPView).