Print from an iDevice with AirPrint Activator (it's free!)
AirPrint Activator is a free download, although the developer politely asks for a small donation, making it a much more inexpensive option than the $20 Printopia. It looks very similar to Printopia, with an on/off switch on the left and a pane on the right showing the available printers. And like Printopia, it does not require you to install an iOS app. You just need to be running iOS 4.2 or later. Unlike Printopia, however, it does not feature virtual printing to your Mac.
As long as everything is connected to the same network, AirPrint Activator should find your printer automatically, whether a printer is connected via USB to your Mac or via a wired or wireless connection to your Wi-Fi router. Unless you have a Mac Mini and always leave it running, one drawback is that your Mac needs to be running and connected to your printer in order to print from and iDevice using Printopia.
I was able to print to my old USB-only Canon MP500 printer from my iPad 2's Mail app, Photos app, Web pages in Safari, and iWork apps Pages, Numbers, and Keynote. If you don't see you printer listed, make sure Printer Sharing is turned on. Go to System Preferences > Print & Scan and click the Sharing Preferences button. On the next window, make sure Printer Sharing is checked. Also, for your chosen printer, make sure to check the box for "Share this printer on the network" on the Print & Scan page.