Printer Pro by Readdle has been out (in various forms) for a very long time. I used it myself before I discovered Printopia. It requires two components to work with USB printers. One is an app on the iPad, the other is an app on your computer.
Printopia and OPrint do not have an iPad app. They use the built in AirPlay ability of the iPad. On the computer the app pretends to be an AirPlay compatible printer, or printers. Because there is no iOS app, you won't find them listed in the App Store. You have to search on the internet to find, download, and install them.
Printopia has been out almost as long as Printer Pro. OPrint is more recent, but there was another (no discontinued) app for Windows before that.
Nothing wrong with Printer Pro. For some uses it can even be superior; but I like the flexibility of Printopia better. The ability to print to folders can have some interesting byproducts. For instance, if I set up and print to a folder in my Computer's DropBox folder, I can upload a PDF of the file to DropBox in one step. {AirPrint outputs as PDF or an image file, depending on the document}.
That's not as useful as it used to be, with all the great PDF tools available today. Still, it's cool.
The thing I still do is print email and web receipts to a folder (named Receipts) on my computer. It's a quick, easy way to make sure I have a record of those transactions.