This is my favourite colour contrast checker. Not only does it provide you with an array of WCAG compliant colours when the pair you tested fails, it also has a creative sense of humour!
This desktop application by Paciello Group allows you to sample colours from anywhere on your screen (PC version only) and check for WCAG compliance.
*Note* The Mac version is awful. Since I last checked, you cannot sample on-screen colours with the Mac version.
The Colour Blind Checker by Etre allows you to upload images to see what they may look like to a person with colour vision deficiency. You can select different deficiencies from the drop-down menu to get an idea of what the image would like to various different people.
Free downloadable application to check image file or website for it’s physical properties in RGB or CMYK and visual disabilities and anomalies. It is available for Windows, Linux, and Mac in English and Japanese.
Have another automated colour checker to recommend? Feel welcome to comment below!