Sure you can buy yourself a pre-made tape emulator for your C64 or Vic20 like the Tapuino or even modify a Commodore Dataset so you can load tape images right off your iPhone via audio files, but where would the fun be in that?
If you have a Raspberry Pi, an LED, a few resistors, an NPN transistor and some jumper wires laying around, you'll be ready to emulate those painstaking slow loading cassette games. Frankly, if you read this blog you should have all of these kicking around.
The project works just fine on an original Raspberry Pi 1. With the Raspberry Pi already going through multiple revisions, I have a drawer with 3 or 4 Pi 1's just begging to be used.
You don't require any special software for your PET/C64/Vic-20, you load and save PRGs to the Pi's SD card just as you would to tape, it even allows you to traverse directories and delete files. Don't expect super fast I/O as there is no fast loader currently implemented. Then again, if you are screwing around with a PET then you really can't expect super fast read or writes.
You can find everything you need to get started on RhinoDevel's GitHub page