OK, tell me if I am understanding this right...
My laptop is already wifi enabled but I would still need to purchase the aircard. But doing so would enable me to access the internet for "free"?
No no no...
Wifi and aircards are two different beasts.
Wifi, which is now built into most laptops and can be added on via PCMCIA or USB adapters to others, is a short-range wireless technology, used at offices, homes, and even coffee shops to allow network access. You have to be in range (typically 300 feet but it can be more) to use it, and you must be permitted to use it (have the right security code, pay a fee, etc.).
An "aircard" is like a cell phone for your laptop. Think of it as a dialup device, except that it can (not always) go a lot faster. It's still wireless, but works over the cellular network - so it can work in a larger variety of areas, usually anywhere your digital cell phone works.
Neither is generally free. The aircard has at a minimum a monthly fee (it used to be $99/mo, but someone else here said $59/mo, and that could depend on the carrier - I know Verizon and Sprint offer them).
Wifi access depends on who is providing it. Some coffee shops offer it for free (subsidized by the coffee prices

). Hotels/resorts may use it instead of wired connections, but you still have to sign up with a fee to access the Internet. But you must be in the local area it is offered at. A Doubletree I was at recently wanted $12 to let me use it.