This article has a wealth of links to web design tools and utilities you WILL find useful. These are tools I have found, and use myself, not ones I have created! All these tools run in the browser, and don’t require you to download any files.
Colour scheme tools
Online color scheme generator 2
http://wellstyled.com/tools/colorscheme2/index-en.html
This is a wonderful tool you can use to make your own colour schemes. You can set up a scheme based on different models (e.g complements, see my colour schemes article).
Another great feature is that you can see your scheme from the perspective of people with colour disabilities; this aids you in finding a scheme that is accessible!
Color Schemer online
http://www.colorschemer.com/online.html
Not as feature packed as ‘color scheme tools’, but can help convert colors from RGB and hex, and shows colors that fit with the selected color.
CSS tools
Flumpcakes css optimiser
http://flumpcakes.co.uk/css/optimiser/
Excellent online tool for optimising and tidying up your CSS files. You simply plug in your css file, and it outputs a cleaner version, and also displays the amount it has cleaned up so you can compare file sizes. The only drawback is that it will remove any hacks you have added to your css file.
Validation
W3C CSS Validator
http://jigsaw.w3.org/css-validator/
This is the bog standard css validator from the W3C. Make sure your code validates, then you can also have a little icon to put on your site to show this!
W3C HTML validator
Probably the most useful tool in this listing. Allows you to validate your html code. This is essential for making sure the page will render right, and clearing out bugs.
WDG HTML validator
http://www.htmlhelp.com/tools/validator/
Very similar to the W3C validator.
FEED validator
Validate your RSS feeds to ensure users will have no problems viewing them..
Accessibility checkers
Watchfire
Validate your WebPages and highlights flaws with the accessibility.
Compatibility testing
Safari test
http://www.snugtech.com/safaritest/
Great tool for testing Mac compatibility. You just add your site url, and the tester renders an image of your site in safari, showing you want mac browsers will see. This tool is also useful for generating full screen page screenshots!
Misc tools
Button maker
http://kalsey.com/tools/buttonmaker/
This tool allows you to create cool little buttons for your site. You can customise the text and colours and output to a file.
Web 2.0 validator
This tool allows you to check how web 2.0 your site is!. Quite entertaining to see the results.