This is the text frequently displayed in bold lettering on self-serve compressed air kiosks at various fillup stations. These kiosks allow customers (and sometimes non-customers) to inflate their tires to the correct pressure. Free air stations range from the minimalist interpretation to fancy, attractively colored metal enclosures. Inside there is a small air tank and an electric air compressor. The attached rubber hose should be sufficiently long to reach each corner of a parked car, allowing for ease of use. The hose is stored on some kind of spool, spring loaded or not.
Now naturally, the first impression one gets upon seeing the phrase "Free Air" is one of distinct irony. Of course air is free! Who in their right mind would pay for air! Well, if you are unlucky enough to have a dangerously low tire in an area where there are no free air stations nearby, you will be forced to resort to using one of their stingy coin-operated cousins. This usually leads to profuse swearing when the machine switches off in the middle of pumping up the third tire. The opportunity to fill up for free is a blessed chance.
The generosity of providing free compressed air is apparent when you consider that doing so is not without expense. The initial investment in equipment and materials alone must be substantial. Throw in ongoing maintenance and electricity to run it and you can appreciate the proprietor's magnanity. Something I appreciate even further is that most of the time it is made available to customer and non-customer alike.
Here's a tip for the next time you are wondering where you can fill up a tire and don't have any quarters handy. Look for a Chevron or other top-tier gas station. That's where I have the best luck. Chains that cater to the low income crowd much less so. 7-Eleven almost always has compressed air, but for a price. If you're just not having any luck, head for a quick lube joint or a tire shop, and hopefully they will be nice enough to help you out.
Remember to keep your tires properly inflated. Check the tire manufacturers' specs for the correct pressure. It can be found on the sidewall of your tire. Poor gas mileage and uneven tire wear are some good reasons to pay attention.
DejaMorgana has pointed out that, in some areas, free air stations will not function unless you press the "Super-Sekrit button" on the bottom. Good point if you can't seem to get one to work.