Color Preview Applet http://members.shaw.ca/simonwigzell/ColourPreview/
Define colors by entering RGB or Hex values directly or by manipulating sliders for both foreground and background. Colors are automatically displayed, and all color definition methods are automatically updated.
Forestasia.com http://www.forestasia.com/tools/colour_converter.asp
Select the hexadecimal color code from the drop down lists, or enter RGB values in the text boxes, and see the converted value calculated and displayed in the textbox below. The background of the textbox will preview the color selected.
RGB Color Calculator http://www.drpeterjones.com/colorcalc/
Convert between 0-255 RGB values and HTML hex code dialup color calculator. Can use several kinds of increments for changing the red, green and blue settings. Includes brief RGB and HEX tutorials.