Usually, Apple requires you to buy an AirPrint compatibile printer to allow your iPad or iPhone to print.
However with this simple trick, you can make any printer connected to your Mac and shared over the network, into an AirPrint printer.
Step 1:
From System Preferences remove existing printer(s) you have setup.
Step 2:
Launch AppleScript editor
This is usually found in Applications > Utilities > AppleScript Editor
Step 3:
Copy and Paste the following into the Editor and Press 'Run'
Code:
do shell script "echo '# enable AirPrint
image/urf urf (0,UNIRAST<00>)' > /usr/share/cups/mime/airprint.types" with administrator privileges
Example:

Step 4:
Restart your Mac
Once its restarted, add the Printer as normal and enable sharing over the network, with permissions set as default 'Everyone'
Step 5:
Open up your iPad to a webpage in Safari, and click the button next to the address bar and select your printer (see below)

Enjoy! 
Tested on iPad 2 running 4.3 and Mac OSX 10.6.7 - worked perfectly with a old Dell Laser Printer.
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