Lately I have started using a neat tool to help test web applications: http-console. As you will see from its page, it basically provides a command-line interface for speaking HTTP. Thus far it has been a great tool for testing out APIs. Using http-console is not a substitute for ‘proper’ test suites—I like PHPUnit for that purpose—but it is proving to be a useful complement. If you are writing any kind of web application that offers a public API, say something based on REST perhaps, then you may like http-console.
Web developers should download it and try it out. And if you know of any similarly useful tools then please let me know here in the comments.