The iPhone's native text messaging tool has forwarding capabilities, but third-party apps are out there that can do it just as well, and sometimes with added features. A number of robust text messaging apps are available for download from the iTunes Store, and many are free.
ITim Text MMS Messenger
ITim Text MMS Messenger is a free SMS and MMS client for the iPhone, iPad and iPod touch. In addition to text forwarding capabilities, its features include Google Voice Push alerts, Facetime MMS compatibility, an integrated media gallery and customizable notifications. The app also lets you send videos, sounds and animations with your forwarded text messages. ITim Text MMS Messenger is an 8.5MB download, costs around $4 and requires iOS 3.0 or later, as of the date of publication. Tto use the app's forwarding features, you must have Apple's iOS 2.0 or later installed.
Textie Messaging is a text messaging application that lets you communicate with other Textie users for free. Its message forwarding feature requires that you have iOS 3.2 or later installed on your iPhone. Additional features include threaded view messaging, archived message loading and the ability to add photos to your text messages. Textie Messaging is 2.0MB and requires iOS 3.1 or later. It's a free application supported by ads. A paid version without ads is available as well.
GV Connect is a Google Voice client for the iPhone. The application lets you forward SMS messages to other mobile phone numbers and to email addresses. Text messages are displayed in threads, just like in the native iPhone texting application. Additional features of GV Connect include message archiving with tags, a message history search tool and full integration with your iPhone's contact list. GV Connect is a 1.9MB download, costs around $3 and requires iOS 3.0 or later. You must have a Google Voice account for the app to function properly.
TextNow is a free text and picture messaging application with support for all major U.S. phone carriers. In addition to standard text-forwarding capabilities, TextNow lets you set up a separate phone number with text messaging support, voicemail and voice message forwarding. The free version of TextNow is supported by ads. A paid version without ads is available from the same developer. TextNow is a 19.3MB download that requires iOS 3.0 or later.