Printing remotly from your iPhone or IPad using an app
FingerPrint (OS X &amp; Windows) is a paid application that enables a printer connected to your Mac or PC to become an AirPrint printer. It is a paid application.

For Windows

There is also a free program for Windows. You can download the files here. One they are downloaded, unzip the zip file and run the AirPrint_Installer.exe Then run either AirPrint iOS 5 FIX - 32Bit.reg or AirPrint iOS 5 FIX - 64Bit.reg depending on if you are running a 32 or a 64 bit version of Windows. Go back to AirPrint_Installer.exe, make sure "Service Startup" is set to "Auto" and click "Start". Reboot your computer. All these instructions and troubleshooting instructions can be found in the links above.

For Mac

AirPrint Activator is a free application that allows you to turn any shared printer into an Airprint printer. I have used it quite a bit now, and I highly recommend it. It has a very simple and easy to use UI: