I use Color Schemer by color schemer dot com and I'm really very happy with it (altough it's not free AFAICR)!
Have a look at Pixie http://www.nattyware.com/pixie.html It's a very popular 8k free software. Also my favorite.
I use the online version of the "color schemer". It can be found at http://www.colorschemer.com/online.html. Has some limitations but has been of good use to me
I offer a download for a basic color picker on my site Here. I keep it on my desktop. Not a greeeat eye for color myself, so I use it regularly.