Thankfully, there are a few apps that will let you print wirelessly to the printer you currently own, whether it's a Wi-Fi printer or USB-only printer.
Windows users can turn to FingerPrint, a $10 utility that works with both PCs and Macs.
For $20, Mac users can use Printopia, a small app that cleverly installs itself as a simple pane in System Preferences. Like the aforementioned apps, it lets you wirelessly print from an iOS device. In addition, it also lets you use your Mac as a virtual printer; instead of to the printer, you can send it as a PDF to your Mac.
For even more control, there is an iOS app that acts as a printing hub for your iPad. PrintCentral costs $8.99 and lets you print documents, e-mails, Web pages, photos, and more, all from within the app.
Lastly, you can print wirelessly from an iOS device using Google Docs.