PHP

I use this trick all the time, practically every day in fact when I'm building a website. When you need to test a PHP snippet or you want to find out how something responds -- you want to see the structure that taxonomy_get_tree() returns, for example, or you forgot what the structure of the data returned from a multi-select form element is -- you can use this shortcut instead of going through the whole tedious process of writing a module or running the risk of blowing up your site by saving the PHP in a node.

I was recently asked to download a series of images from a remote server. The image URLs were not known ahead of time, so to figure them out, I had to submit a search form, parse the results for a way to determine the file name, and then download the images onto the local server.