Here's the problem with AirPrint: not nearly enough printers support it. Plus, there's that whole consumables thing. Aren't we supposed to be living in a paperless society?
Print to PDF for iOS solves both problems, allowing you to "print" e-mails, Web pages, and documents as PDFs right on your iDevice. It's a clever app, one that works much like PrimoPDF and other utilities for Windows.