PrintCentral ($8.99) provides robust printing support that lets you print via Wi-Fi, 3G/EDGE, Bluetooth, and to USB-only printers. If you have a Bluetooth or USB printer, you'll need to use your PC or Mac as a proxy. To do so, you'll need to install EuroSmartz's free WePrint application. Regardless of your setup, your iPad and printer will also need to be on the same network.
I have an old USB-only Canon MP500. I connected it to my MacBook, on which I installed WePrint. On my iPad, I opened the PrintCentral app and set out to find my printer. The app includes a Getting Started doc that will walk you through how to connect to your printer. You can find this doc by tapping the Files button from the bottom menu bar. Then from the left panel, you'll see the Getting Started doc listed. It will direct you to tap the printer button in the upper-right corner and tap the blue Choose button in the printer field.
My printer was listed on the Available list. In fact, it was listed twice. I selected the one with WePrint below it, which is what I was using to connect. Now, by tapping the Print button in the upper-right corner (the same one you used to connect to you printer) and then tapping the blue Print button, you'll be able to print docs from the PrintCentral app.
PrintCentral features a built-in e-mail app and Web browser. You can't use PrintCentral to print directly from the default iOS Mail app or Safari, but you can add e-mail accounts to PrintCentral's e-mail app. And you can easily print Web pages from its built-in browser. These options are listed next to the Files button in the bottom menu bar. There are also Contacts and Images apps and a Clipboard, which you can use to import other types of documents to PrintCentral for printing.
There are other options for printing from an iPad or another iOS device. For Mac users, Printopia and AirPrint Activator will have you printing without needing to install an iOS app. For Windows users, there's FingerPrint, and you can always try printing from your iPad using Google Docs.