So, you don't even need to have a balance on a credit card to be able to apply for a new card and transfer nearly its full credit line to your existing credit card or bank account. You can then put the money in a risk free account for the 18-month holding period and earn about 3% in the current interest rate environment. You will be required to repay 2 to 3% of the principal each month - so you would need to make sure that that amount of cash was unencumbered to meet minimum debt service requirements.
Who would have imagined that credit card companies would get into the business of making unsecured 0% 18-month loans without any fees?