Change the way you write web pages
BlueprintCSS
Building a CSS layout shouldn't be the hard part of your design. Blueprint lets you code the skeleton of your site as quickly as you can type. Then you just add a second CSS file with your graphic design to fit within that layout, and you've saved a bunch of testing time. Also, unlike the other grid kits like YUI Grids, you get a command-line tool that lets you customise the columns, gutters, and most excellently of all, to create semantic references once your layout has settled.
YUI
The Yahoo! User Interface toolkit is a staggeringly comprehensive collection of tools, helpers, design patterns, examples, and libraries to help you get web stuff done. The javascript libraries are of particular note - because they offer access to minified, compressed scripts from their content distribution network, and because the chances are good that visitors to your site have visited some other site that uses the same scripts, the libraries don't put a huge extra burden on your pageload.
Helpers like YAHOO.util.Dom and YAHOO.util.Event make writing unobtrusive, professional javascript a cinch, and unlike some other libraries like JQuery, there's a very "pure javascript" feel to them - you don't feel like you're learning a new language, or something you won't be able to adapt to other circumstances.