The competitor to Apple’s closed and tightly controlled App Store, Cydia Store, has recently opened. It works on only jailbreaked phones, so that constitutes a complicated process for most users. With Cydia Store, developers of jailbreaked apps can also charge for their products.
The advantage of Cydia’s App Store is that it hosts applications that do things ‘legal’ applications can’t do. For example, the camcorder software Cycorder is only available through Cydia. Same applies to applications that let the user do user interface customizations which are not possible otherwise.
Cydia uses Amazon payments to process the paid applications. The ‘Download’ button for free apps turns into ‘Purchase’.
The user experience is not as smooth as on the App Store, you have to go though a web interface to identify yourself and make the payment.
You need to Jailbreak your iPhone to use Cydia Store.