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.
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.
A tool to help you identify fonts you see on the web.
Color Tools
This tool is designed to integrate with Photoshop and will let upload your own palettes as well.
Use this tool to sample colors in an image so that you can match them in hexidecimal or RGB values.
What particular colors can mean to different cultural groups. Fascinating visualization!
Image Resources
This is the optimization tool we'll use so that our images download quickly. It will allow us to resize the image in addition to saving in the proper format.
This is another option, which lets you compress more within the same format.
This is an image-generating tool that you can use for free. It will allow you to do some of the basics in terms of building banners and logos if you don't have access to Photoshop or Illustrator.
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.
Both sites offer free stock art from talented photographers.
CreativeBloq has a good collection of paid services here, with comparisons.
Browsers & Screen Resolution
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
From the W3C. This will tell you if your code is copacetic.
Also from the W3C. This will check out your Cascading Style Sheets.
One more from the W3C. A tool that checks your links.
This will let you check your pages for web accessibility problems.
Please make suggestions!
If you find something really useful, please email me and I can add it to the list.