Khoi Vihn, the design director for NYTimes.com, is a fan of 960.gs, and has a great resource on his personal website as well: a presentation that he did on rapid prototyping at SXSW.
960.gs is particularly wonderful because you can download a custom created CSS file based on your column count, and 12 and 16 column grid files to use in pretty much any design program you can think of. A drupal theme is also in the works.
If you ever need inspiration or a gentle prod in the right direction when it comes to colour schemes and palettes, Colorlovers is the website to check out. So many beautiful palettes to gain inspiration from, either just by browsing, or by searching by hue, name or hex number.
Check out their application, ColorSchemer Studio, as well.
A great resource if you're trying to get the text layout of your website to be readable and wonderful. Compare different typefaces, font sizes, leadings, colours and columns. The amount of combinations you can create here is amazing.
w3schools offers a tonne of tutorials for almost everything related to web development. If you're ever curious about what a particular CSS property does, you can probably find out here.
You can also validate your HTML and CSS here.
This website lives up to its name. There are a lot of tools here available for designers and among the best are browser elements you can download for prototyping, wireframing and mock-ups. While you're at it, you can also generate your lipsum here to use until you come up with some exciting copy.
A bonus - If you can, take some time to watch a few. They are always fascinating and inspiring - and great if you can't afford to go see Chris Anderson when he is in town.