HandyPrint is one such application for Mac OSX that enables you to use non-AirPrint compatible printer from iPod/iPhone/iPad, running iOS 4.2 or above, via Mac.
Install HandyPrint on Mac and ensure that the Mac and iOS device are on the same Wi-Fi network. Any printer connected to the Mac will now be accessible from your iDevice! There is no need to install any extra app on your iPod/iPhone/iPad; however, the Mac must remain turned on for the attached printer to be available to the iOS device.
HandyPrint is donationware and is available for free trial download.