This is a list of the best frameworks, libraries and toolkits I know about for the web in 2006. Obviously, this is entirely a personal opinion but these codebases have annoyed me the least while giving me the best outcomes:

Drupal (PHP) - A flexible content management framework with robust security, modularity and taxonomy features. More effort to get a working solution but often easier than taking an "out-of-the-box" product that really isn't designed to be tinkered with ... and tinkering with it.

Think of Drupal as "website lego" whereas a package like Joomla is more of a remote control car in a boxed set.

Dojo (JS) - Highly architected, class-based JavaScript frame for AJAX effects. Still immature and lacking a little in documentation but enormously powerful.

Rico (JS) - Prototype(JS)-based effects library. Easy to use for simple effects.

jQuery (JS) - a new one that I haven't evaluated thoroughly. More or less a direct competitor to the Prototype JS library, but seems to have a good idiom which fits with the JavaScript mindset (and some really cool examples).

Sarissa (JS) - good, solid cross-browser XMLHttp support.