Who should I tip???

Since tipping is one of the most confusing customs, I always doubt when and who to tip. I know about tipping the obvious people such as waiters and barbers but I am not sure if I am supposed to tip other service people, like plumber, cable guy, or electrician here in the United States. In Singapore, service tip is already included in the bill when you eat at a restaurant so there is no need for you to tip a waiter. I always give at least 15% tip to the servers when I eat at restaurants. Depending on their service, I wouldn't mind giving up to 25% tip. They do work hard and some of them really deserve it.