I’ve been using the “Control Suite” for a couple of development projects and I’m 100% pleased. The tools are modular (so you can take what you want), class based (so they don’t break your other code), well documented, fast, easy, and clean.
Anyone who works with/develops AJAX and a dynamic web-user-interface… especially if you already use prototype… I highly recommend you try this set of tools.
Control Suite : High Quality Controls & Widgets for Prototype