Developer: Quickoffice Inc.
Price: $19.99 (on sale for $12.99)
Quickoffice allows you to create and edit Word documents and Excel spreadsheets on the iPhone. To transfer files back and forth between your PC or Mac and iPhone, you can drag and drop via Wi-Fi by mounting your device as a wireless network drive. You can also e-mail files out as attachments directly from within the app, but the ability to import attachments from e-mails received in the iPhone’s native Mail app isn’t yet supported. The app also allows you to view PDFs and images in various formats, as well as listen to MP3s and watch video files.