Cash is so old school. And so is that flip phone you have. So why not upgrade to an iPhone with Square’s card reader and solve both your problems? Square, Inc, a company founded by a cofounder of Twitter, is the first company created to bring credit card scanning to the masses. With a small, free, and little device, anybody who owns an iPhone, iPod touch or iPad can turn it into a mobile cash register. It’s quite amazing.

On Friday morning of MacWorld 2010, Guy Kawasaki showcased five iPhone/Mac developers and gave each one a chance to show off the innovative things they are doing. Guy is a former “Evangelist” for Apple and is currently the co-founder of Alltop. Let’s take a brief glimpse at the apps and developers that were showcased: Square, Hearplanet, Boinx TV, Bing and Bill Atkinson PhotoCard.

Jack Dorsey, the inventor and chairman of Twitter, has just announced his new startup: Square. Square is aiming to be the ultimate pay-on-the-go credit card system for the iPhone, requiring only a small credit card slider that can be plugged into any device with an audio input jack. Basically, using the powerful abilities of the iPhone SDK and ability to interact with third party hardware, SquareUp will allow iPhone users to make their phone the ultimate payment portal.