Color scheme
is a beautiful colour picker. Match a base colour to others with
five different colour match types
(contrast, analogous colours, ...). View your scheme in a
simulation of various forms of colourblindness. Even has the 'web
safe' palette of yore.
The tool design is great, very simple and clean (other than the color
swatch display; more functional than æsthetic).
I love the implementation: a single 'live' web page running
a bunch of complex javascript.