ideone.com : Ideone is an online compiler and debugging tool which allows youto compile source code and execute it online in more than 60 programming languages.
Tabnine : Tabnine is an AI-powered code completion extension for IDEs
mail-tester.com : Free online service that allows you to test your emails for Spam, Malformed Content and Mail Server Configuration problems
Yellow Lab Tools : Online test to help speeding up heavy web pages
Powerful and simple online compiler, IDE, interpreter, and REPL. Code, compile, and run code in 30+ programming languages. including JavaScript, Python, Ruby, Java, Node.js, Go, Scheme, C, C#, C++, Lua and many more.