This is awesome - it's always great to find/discover tools out on the web.  Even ones that have been out there for years and you just never knew about 'em.  This is one of those cases for me.

ColorBlender is a free online tool that will automatically create palettes of matching/coordinated colors (a blend).  You can save your blends, download them in an .ACT file for Photoshop (awesome...I'll be using this all the time), an .EPS file (same for Illustrator), do PANTONE matching, etc.

It has a great UI for managing the RGB values of your color via sliders and provides some slight variations on your color.

For all those out there that are color-scheme challenged, this site should be on your 'do-not-misplace-this-site' list.  Very cool.