Sessions.edu Colour Calculator
While searching the web I came across this example of an interactive colour wheel that can be used by designers to generate colour harmonies for use in design projects.
You first select a design and then a shape (square, triangle, etc.) to rotate around a colour wheel. The resulting colour values are listed as CMYK, RGB and HTML web-safe colours and you can either copy, download or email the number values. You can adjust the saturation and lightness of the hues.
It appears to be very straightforward and a quick tool for generating colour schemes.
An additional feature I would like to see is an option to create harmonies from an existing colour. It would be useful if you already have a colour, for example in a logo, and are looking for colours to complement it.