I sat for hours in Design 101 trying to get a even coats of cadmium red, cadmium yellow medium hue, and pthalo blue acrylic and their various combinations in perfect circles to learn the basics of complementary colors and other ins and outs of color theory.  Now several years later, it all comes back to haunt me with Adobe's Kuler.  I've seen several for-pay downloadable apps to do this, but Kuler is free and web-based (Flash).  Simply pick from a library of pallets or generate your own using a variety of color relationships.  Use with Firebug for quickly knocking out CSS color schemes.

FireBug is an amazing web development plugin for FireFox.  Like Web Developer Plugin, it lets you tweak HTML, CSS, and JavaScript on the fly.  However, it also bakes in features like visualizations to make sense of CSS and deeply nested elements easily.  It also gives you a nice drill-down report of network traffic, so you can see exactly how much each element contributes to the total page size, which is great when you're trying to trim those extra Kb off your pages.  The UI is very well thought out and easy to understand. 

Hang on to Web Developer Plugin for the validation features, but this is definitely a must have for your web development toolbox.

http://www.getfirebug.com/

One of the more exciting things going on in the Microsoft development world is the upcoming release of Silverlight.  Silverlight is (or soon will be) Microsoft's answer to Adobe's Flash and Flex products.  Silverlight technology is powered by the .NET framework in the form of a trimmed-down CLR (CoreCLR) that runs on both Windows and Mac -- Scott Hanselman has a pretty good summary of the product and its associated jargon here.
Based on the early examples, I think Silverlight will quickly achieve the ubiquity that Flash has enjoyed; hopefully it will stir the interest of .NET developers who have avoided Flash in the past and drive the development of some really useful applications.  There's also a good DotNetRocks podcast available in which Brad Abrams discusses the philosophy behind Silverlight and gives some glimpses of its future direction.

See the difference between these two icons?  Me neither, but the one on the right has about an 8% smaller file size. Portable Network Graphics (PNG) is quickly becoming the prefered format for web graphics.  It supports alpha transparency for nice blendy graphics (now in most browsers), and has amazingly small file sizes.  But those small file sizes can be made even smaller thanks to some custom compression tricks and pulling out unneeded data.  PNGOUT is a great little command-line utility for doing just that. 

Since it's a command line app, it can be integrated easily into your build process to automatically compress any PNG files in your images folder, so that no matter where your PNGs come from, you can ensure they take up as little bandwidth as possible.