Postman is HTTP client/API tool to help with designing or working with web APIs.
JSON Editor Online does what the name suggests. Simple editor with the ability to nicely format your JSONs and let you browse the JSON in a tree hierarchy.
RegExr is here to help with regular expressions.
Linux & System administration
explainshell provides explanation to given command lines. It will visually decompose your command line into commands and show you help for each of them. Especially useful when you need to understand a complex shell one-liner.