If your passport is expired, you will not be permitted to travel internationally. Most countries offer expedited service if your passport is expired and you are traveling soon.
If your passport is expiring within six months: keep in mind that some countries will not admit visitors with passports expiring within six months or will not put visas them. So if, check with your destination and renew accordingly.
For U.S. passport questions, visit: http://travel.state.gov/passport/