Featured Color Tool: Adobe Kuler, kuler.adobe.com
Adobe Kuler not only helps you find matching colors, but also offers color themes you can browse. You can also grab colors from an image you upload, and find matching colors depending on the mood you wish to portray, or you can also simply create a brand new color set from one starting color.
ColorScheme Designer
Similar tool to Adobe's Kuler, with some very nice features, like the HTML preview. You can also export your color scheme to various formats.
Color Blindness Simulator for Websites
Check to see what any website looks like to a color-blind person.
Color Blindness Simulator for Images
Check to see what any image (via image upload) looks like to a color-blind person.
Name That Color
An application which shows you the name of the selected color or let's you browse colors by name.
Colour Check
Check to see if a background and a foreground color have sufficient contrast to be readable to all readers and on any and all monitors.