With 15,000 apps on the digital shelves and multi-millions sold to iPhoners, we think it's safe to say Apple is pionnering the field on third-party software. Every now and then, we peek in on the App Store and we're always impressed with the fresh offerings we find. Not only are developers tackling  functional problems like how to work with the iPhone's tiny on-screen keyboard (shout out, Easywriter), but productivity apps like the mortgage calculator we found while digging up the best paid iPhone apps and super-games like Payback, a Grand Theft Auto look-alike for the Jesus Phone, are uber-impressive as well.

Of those we just mentioned only Easywriter is included in the apps of this roundup. Find out what other cost-effective apps are keeping it company below.

Interested in more apps for more mobile operating systems? Check out The Best Free Smart Phone Apps (And A Few Worth Paying For), our list of free and paid applications for Windows Mobile, BlackBerry, Apple, and Android devices.