216 Color Square
This is a clickable imagemap. Click on the color you want, and the hexadecimal number for that color will be returned to you. These colors can be used with HTML documents for backgrounds, text colors, and link colors. (See the Netscape page on Controlling Document Backgrounds for instructions on how to use backgrounds and text colors.)
This version includes the standard palette of 216 colors that are common between Mac and PC platforms. The hex values should be the true color on either platform, with no dithering.
A more elaborate color picker can be found at Visibone's colorlab; they also sell printed color reference charts.