Online tools

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

General Tools

W3C Schools HTML help pages

W3C Schools CSS help pages

HTML5 Cheat Sheet

CSS Mega Cheat Sheet

 

Typography

CSS Font Stack

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.

Real Time Browser Resolution Stats

Collects data from 2500 sites, and more than 300,00 visitors for some interesting information.

Validators

HTML Validator

From the W3C.  This will tell you if your code is copacetic. 

CSS Validator

Also from the W3C. This will check out your Cascading Style Sheets.

Link Checker

One more from the W3C. A tool that checks your links.

Wave

This will let you check your pages for web accessibility problems.

Other Useful Tools

WebsiteOptimization.com

More services to measure download times and usability issues.

Web Page Analyzer
A more in-depth measurement of a page's speed. Cool service. (Free)