Colour contrast checking tools to improve the accessibility of your design

The list:

• Colour Contrast Check: The first really intuitive online tool for checking colours that I know of, Jonathan Snook's Colour Contrast Check lets you input foreground and background colours either by entiering hexadecimal values or by adjusting sliders. The tool gives you immediate feedback by showing what the chosen colour combination looks like and whether it passes the test.
• Colour Contrast Analyser: Gez Lemon's Colour Contrast Analyser uses the same algorithm as the Colour Contrast Check tool, so the results should be identical. The difference is that this tool has no live feedback and no sliders, meaning that you will need to enter the values for foreground and background colours into a form and submit it, after which the results are displayed.
• Colour Contrast Visualiser: Using a slightly different approach, Tom Hooper's Colour Contrast Visualiser helps you find acceptable colour combinations by visualising the colours that provide sufficient contrast on a colour palette. Colour Contrast Visualiser is available in Flash and Adobe AIR versions, so you will need support for either of those to use it.
• Colour Contrast Analyser Firefox Extension: Instead of having you enter each colour combination manually, Gez Lemon's Firefox extension finds all text elements in a document and examines the difference between foreground and background colours for them. It displays the results in a new tab, complete with previews of the colours. This can look a bit overwhelming, but it's a good way of finding problematic areas of a design. Obviously this tool does not work with images – you'll need an HTML document for it to work.
• CSS Analyser: Another tool created by Gez Lemon, the CSS Analyser does a colour contrast test of your CSS. It also validates the CSS and checks that relevant text sizes are specified in a relative unit of measurement.
• Luminosity Contrast Ratio Analyser: This tool, also created by Gez Lemon, tests foreground and background colour combinations according to an algorithm developed by the Trace R&D Center.
• Vischeck: A colour blindness simulator that can be used online or as a Photoshop plugin that appears in the "Filter" menu. The online tool simulates colour blindness on an image that you upload or on a web page that you specify, while the Photoshop plugin changes the colours of the document you are working on.
• Contrast Analyser, Version 2.0: A standalone application available for Windows and Mac OS X. This is a very neat application that does pretty much everything except analyse a web page for you. You can specify the foreground and background colours numerically or by pulling sliders, you can set it to use the colour/brightness or luminosity algorithms, and it will check and preview the selected colours against various types of colour blindness.
• Graybit: An online tool developed by Jona Fenocchi and Mike Cherim. This tool does what you can guess from its name – it lets you submit a URL to a web page which will be converted to grayscale and displayed in your web browser.
• Sim Daltonism: Available for Mac OS X only, this standalone application does not calculate colour contrast, but is still very useful. It simulates various kinds of colour blindness in realtime, which makes it easy to check if a design is likely to cause problems.

All tools mentioned here are free, so try them all to find the ones that suit your setup and workflow best. I don't think it makes a lot of difference which tools you use, as long as you do use one that calculates the contrast ratio and one that simulates colour blindness.