I'm not sure how this one slipped by me but I stumbled across it yesterday by accident whilst looking at an email attachment on my iPad using the Gmail app. - Google Cloud Print. This lets you print from any device to any printer you've shared via the cloud. You can print from you iPad or Android phone to your printer at home or the office printer. Moreover, you can share a printer with family or friends; a friend could for example print a photo on your printer from anywhere in the world. Some new printers are "cloud ready" but you can also configure old school printers using Google Chrome. This is Cloud Print's big advantage over Apple's AirPrint in that you don't need a new printer to use it. I have to say I'm impressed. Setting up my printers following Google's instructions was easy and I can now print documents from Gmail or Google Docs on my iPad - very useful.
[Note: if you have a Mac installing the free handyPrint program on your Mac let's you share any printer to any iOS device across your network.]