Depends, to simply 'chill out', anything except Thursday, At The Drive-In and Funeral for a Friend will do. To calm down if I'm stressed, it's rather antitethical, as those three are the best. Get so hyped up, jump around screaming and yelling for half an hour, and I'm simply too tired to be angry or depressed.

For chilling though, DDR music usually does it, or FFVIII stuff. Some female artists like Sarah McLaclan and Michelle Branch work well, too.