Today I'll be doing a one-way rental that would have cost about $100 for a day. Instead of using $, I used a free rental day. National's 1-2-free gave me several days of free rentals that can be used for one-ways. Outside of that it's one free day for every 5 rentals. I'm sure other car rental programs have similar deals.

Of course this is only useful if you already have the free days (or time to accumulate them) and are doing a short-term rental.