Square is my second-most favorite creation by Jack Dorsey — who, if you didn’t know, also co-founded Twitter (which is, of course, my first-favorite). Square is a mobile credit/debit card reader for iOS devices, and along with the app, you can use Square to personally accept credit and debit cards anywhere you go. The Square reader is small (1/2 inches on each side), plugs into your iPhone (or iPad) headphone jack, and after quickly setting up a free account, you can accept credit and debit card payments.

Because of its ease of use, Square has become popular among individuals and more tech-oriented small businesses to revolutionize how we transact business. College students can use Square to pay each other back for rent and utilities, and it’s a popular way to avoid splitting the bill at restaurants — instead, the other person (or people) can just hand their debit or credit cards over to the one who foots the bill. Earlier this year, we looked at other emerging ways individuals and small businesses could use Square.

Until recently, however, the reader required to use Square was only available via Square’s Web site after signing up for an account, and took a few days to arrive at your home or office. (Square has also been handing out readers for free at trade shows and conventions.) Now, the Square reader is available at stores including Apple, RadioShack, Best Buy, Target, and Walmart. The reader costs $10 — which is $10 more than the free reader you can get via the Web site — but allows you to start using Square almost instantly.

The prevalence of Square readers for consumers will likely disrupt how businesses themselves process transactions as the method becomes more acceptable, compared to current registers and POS stations. Like customers of Apple already experience, it’s possible that the future of checking out of a store, or even restaurants, will no longer require waiting in line at a cash register or waiting 15 minutes for your waitress to return with your credit card. The Square reader could also help smaller businesses just replace their existing register system — even if they want to stay at the front counter.

Have you started using Square to accept payments from friends and family? Would you like to see more businesses using this type of service for payments? Let us know what you think in the comments.