Within a very limited range you can increase the slip of the motor by reducing the voltage. It will cause starting problems and you can not reliable set the speed. Generally not a good idea.
It depends on what you mean by an AC motor. 3 phase motors, definitely not. As you reduce voltage you reduce torque and if the load requires the same torque, the motor slows down. This however increase slip, which causes the motor to require more current as it tries to regain synchronous speed. If it can't because of the load requirements, the increased current overheats the stator and rotor and the motor burns up. If you change the frequency at the same time as the voltage, you can change speed without overheating the motor (oversimplified). This is what a Varible Frequency Drive does.
1 phase motors are a different story because they come in many types and designs. One of them, a "Shaded Pole" motor, can be controlled with a rheostat very successfully, that is typically what a ceiling fan is. A "Universal Motor" such as those found in a portable drill APPEAR to be an AC motor because the DC conversion is done inside the unit, and they too can be speed controlled from the outside with a rheostat, although most already have speed control as part of the system anyway (that is why they used that kind of motor). Any kind of capacitor start motor however will not work well as pointed out by OperaHouse, and they comprise the vast majority of off-the-shelf 1 phase AC motors.
"Venditori de oleum-vipera non vigere excordis populi"
Some induction moto0rs are specifically designed to be speed controlled by variable voltage control. Thes motors have a high resistance rotor and a generally an integral part of fans systems. There is a considerable amount of heat dissipated in the rotor of this type of motor, so the efficiency is down and the motor needs plenty of cooling.
You can not vary the speed of a standard cage typ induction motor with variable voltage as this will cause a very high current and motor damage.
Best regards,
