Android CUPS Print
This app provides a Print Service to Android. This means that once it's installed, you need to enable it in the 'Print' section of the settings app. Once the service is enabled, printers are automatically discovered using the mDNS protocol. You can print anything you want from any app, as long as the print service is enabled.
Of course, printing document is an important matter. Because your documents are your most private information, you don't want anybody to have access to this information. This is why this app is completely open sourced and free to use, modify and redistribute.