I’ve been meaning to post this to my blog for a while:

Easy RGB Color Harmonies calculator
http://www.easyrgb.com/harmonies.php#Result

Just enter your color in the boxes provided.  You can use either decimal (0-255, or 0-1) or hexadecimal (00-FF) values.  On the next screen, you’ll get a grid of complimentary colors based on the color you provided.  Color values are provided in decimal and hexadecimal forms.  This is a really great tool for getting color ideas for web projects.