I will assume you meant 120VAC motor as 'classic'
Then maybe a VFD would allow you to change the speed. I know they use them for three-phase, but I think there is some that will work on single phase.
http://www.anaconsystems.com/text/opti_e2.html
Your second option is to use a DC motor and use PWM to control the speed. This is actually quite easy, but the motor will cost more then a AC equivalent.
I would not use a stepper motor. But, you haven't exactly told us the job the motor is performing or how big the motor needs to be and whether or not it needs to change directions.