I know this works at BWI and I am sure it works at other airports as well. If you go to the curb outside (where people get ground transportation) prior to going to baggage claim, you will see several abandoned luggage carts. If ground transportation is too far away, it may not be worth saving the $3.00.
I frequently go out and grab one at BWI instead of paying for one. I actually got this tip from someone that worked at the airport one day when they were all out of carts.
The Travel Ringer