http://tinyurl.com/pp8z9
This particular feature (on www.easy2diy.com) impressed the heck out of me. If your planning on jumping in on some home improvement, and tackling a painting job, this can make things easier.
Point your browser to the "tiny" URL above. You'll find a template which asks room dimensions, number of doors and windows, and will translate that into how many gallons of paint you need.
If that's not handy enough, it will let you compare various color schemes to decide what colors will look best together and on the room.
If you're going to paint, check this out!