Color Tools

Kuler Color Designer
Kuler allows you to pick your color scheme now too, with a really nifty interface to boot. And if you just want to look around for a color scheme that someone else has already designed, you can use the explore option. These palettes can be dropped into Photoshop directly too.

Color Scheme Designer
This one lets you pick by the kind of palette you want -- complementary, analog, etc.

Colors in Different Cultures
What particular colors can mean to different cultural groups. Fascinating visualization!

Images

I've mentioned the Creative Commons advanced search function at Flickr and DeviantART in class. Also: MorgueFile and for a listing of websites that provide pattern background generators, ipietoon.com.

Or you can use the Search function at Creative Commons, to look for images, video and more.

Embedding Fonts

The easiest way to get started with embedded fonts is Google Fonts, a tool that gives you access to hundreds of open source typefaces. Ensure that whatever you pick for your body type is legible though -- it's open source, so some are better than others.

Font Stacks

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

Screen Resolution & Browser Stats

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 & Accessiblity Tools

Vischeck
For testing to see what your site and images look to people with color blindness

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.

Western University (Accessibility) Checklist
Western has a nice page of common accessibility problems and how to solve them.

 

Page Load Optimizers

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)

Web Page Test
Services to test your website for download times and a variety of other factors. You can set the download times.