With some fancy juggling you can continually apply for no-fee, no-interest, balance transfer cards, and use one to pay off another just prior to the end of the promotional period. Note that if you screw up, interest and penalties can ding you.
Many years ago my B-I-L did it as a proof-of-concept, took a cash advance of 5K on a CC, immediately used a no-fee, no interest balance transfer to pay it off, and kept it going on sequential cards for 8 or 9 six to twelve month cycles (depending on the newest cards terms) before finally using the original 5K to pay it off and stop the ball rolling. He had a zero interest loan for 5 years, and invested the original 5K, making IIRC over 15% profit. It was a game to him, and he had more than enough cash on hand at any moment to pay it off with no penalties if for any reason he couldn't find another card to roll it to.