Online tools

This is not an exhaustive list, obviously. There are so many great resources out there! But it's a start:

General Tools

HTML5 Cheat Sheet

CSS Mega Cheat Sheet

Typography

If you want to use system fonts, and to be sure that your font choices work for a larger percentage of users, check out the lists available here. You can see the availability if you mouse over the "i", and the copied stack will have viable backup fonts included.

Google Web Fonts

Another approach you can take now is to embed the font(s) you'd like to use on your website, and the easiest way to do this is to use the open source typefaces available from Google.

Font Face Ninja

A tool to help you identify fonts you see on the web.

Color Tools

Kuler

This tool is designed to integrate with Photoshop and will let upload your own palettes as well.

Colors in Different Cultures

What particular colors can mean to different cultural groups. Fascinating visualization!

Image Resources

Creative Commons Uber-Search

This tool will let you search the web for artwork which has a creative commons license. Simply click on the site you want to search, and then put in your search terms.

14 Stock Art Collections

CreativeBloq has a good collection of paid services here, with comparisons.

Browsers & Screen Resolution

W3C Schools

In addition to lots of other great stuff, W3C schools has a decent collection of stats on the browsers people are using most commonly and the screen resolutions they use.