These days, Dremel rotary tools are pretty affordable, but if you want one for the price of free you can make one from stuff you likely already have around the house. It may not have all of the power of the real thing, but it will handle simple jobs for you.
This is a simple build that uses a small motor, a 9 volt battery, a switch, some electrical tape and hot glue. The cutting wheel is made from a bottle cap and a pen refill. It will even cut through soft woods like balsa. You can learn how to build your own from the video from Mr. Gear below:
Even if you already have a Dremel, this looks like a fun project to do. Just remember to be safe and wear some goggles and keep your appendages away from the blade. You are spinning a sharp bottle cap after all, even if it is just powered by a 9-volt battery.