There are various standard ac supplies around, aircraft as mentioned which is often 3 phase, also lighting and professional power tools sometimes have a higher freq ac supply.
If you want as simple as possible variable voltage mains fre (50/60 hz) supply then the variac is the easiest option, you can synthesize ac as done in variable frequency motor drives, this can be quite complex though. Another way you could generate various kinds of ac waveforms is to use an audio power amp, this does limit the voltage and max freq though.