Transfs-card-terminal is a project mainly written in OBJECTIVE-C and C, it's free.
An open-source credit card terminal for the iPhone
h1. TransFS Card Terminal for iPhone
This project is an open-source iPhone application for processing credit card transactions via a number of gateways. It is powered by the ObjectiveMerchant library (a port of Ruby's ActiveMerchant).
TransFS Card Terminal is sponsored by "FeeFighters.com":http://feefighters.com. FeeFighters.com is an online marketplace for business financial services, such as "credit card processing":http://feefighters.com, insurance, payroll processing, and business loans.