iOS is all about simplicity, and so Apple has built no-configuration printing capability for it, called AirPrint. The trick is that your printer needs to be AirPrint-compatible — but if you already have one you want to use, there may be still be an answer for you.
Printopia is the more polished and multi-featured application, but both perform similarly, by using your Mac’s printer sharing feature to let your iPad, iPhone, or iPod Touch “print” to your Mac, which in turn prints to your printer (or, in the case of Printopia, can alternately have the Mac save it to disk, email it, or save it on Dropbox). The catch, of course, is that your Mac has to be on for either to work.
Cat on a printer photo by Mary-Lynn, from Flicker Creative Commons.