ColorSmart by BEHR® Preview Rooms
BEHR's website lets you upload pictures of your home, choose a paint color and decide whether you really like it before you start painting. They also offer model rooms that you can "paint" if you don't like the upload option. This is a great tool for matching accent colors to your primary room color or to try riskier colors like reds and oranges. It gives you the opportunity to see it before you buy it.
Autumn Air (top) and Spiced Berry (bottom) by Behr