The prototype javascript library is one of the most brilliant, time-saving tools a web developer can use. It’s so easy to use this in conjunction with script.aculo.us to create some user-friendly “drag-and-drog reordering elements with auto-save” to a page.
I’ve used it in a giant database-driven application we’re developing, and it has helped to trim hours of work down to mere minutes.
UPDATE: My new favourite thing!