The ability to complete mobile transactions with credit card terminals is a great use of the iPhone for employees on the go or companies that do home or office calls. Like most applications in the iTunes App Store, there is no shortage of alternatives available for you to try. Here's a roundup of some of the most popular credit card terminals for iPhone.
Credit Card Terminal [iTunes Link] - $0.99
This app is pretty awesome. So awesome in fact, that it was featured internationally in an Apple commercial. The 99 cent application offers a (very) cheap alternative to expensive terminals and hardware. With a clean and easy to use interface, users can enter credit card information, complete transactions, and even view and refund past sales. The app also gives you the ability to collect customer information. The developer even offers telephone and email support and will walk you through setup.
Billing: Credit Card Terminal [iTunes Link] - $19.99
Another "easy to use" credit card terminal that is guaranteed to help make transactions easier. Sporting a pretty cool, and somewhat different kind of interface, the $20 application makes setting up a sale as simple as tapping a button. One cool feature offered in this version is the ability to get a customer's signature.
Some screenshots from the featured applications: