Mac OS X: You could use AirPrint Hacktivator (or do it manually) to add AirPrint support to your Mac, but if you don’t want to modify your system files and would like virtual printers, you should check out Printopia.
Printopia is a simple preference pane app that lives in System Preferences and allows you to send print jobs to your Mac from your iOS device. It works as simply as checking a box next to the printers you want to share with local iOS devices.
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Additionally, you can enable two virtual printers: one that saves a file to your Mac and one that saves a file to your Dropbox folder. The virtual file can be either a JPEG or PDF. This is great for saving web receipts that you don’t really need to print.
Printopia is compatible with Intel and even PowerPC Macs running OS X 10.5, so you don’t need to upgrade to 10.6.5 for it to work. It’ll cost you $US10, but you can download a demo for free. A slightly cheaper option is Fingerprint ($US8), but it doesn’t provide virtual printing support.