I like colorcharts.org it's a great site but I also use MPC - http://www.myperfectcolor.com/ this site is simular in that is shows you all the colors from all the brands. The major difference is you can buy the paint online from MPC.

My favorite is there color combinations
http://www.myperfectcolor.com/Color-Combinations-Myperfectcolor-com-s/3254.htm

All brands matched by MPC link
http://www.myperfectcolor.com/articles.asp?id=135

I am pretty sure they Use Benjamin Moore to mix all the colors regardless of what brand they match.