If you want to send text messages to friends regardless of their device or operating system, you might want to take a closer look at mysms. Mysms is an online service lets you send, receive and store text messages across multiples devices and platforms. It recently added the iPhone to its growing list of supported operating systems, which also includes OS X, Windows, and Android. There's also a web app that'll work with the iPad and a Facebook extension.
The best feature is its cross-platform syncing, which lets you manage all your text messages from any device. An iPhone owner that uses a Windows Machine can type out a message using their desktop keyboard, send it to an Android user and then check the response from his or her iPhone.
Mysms is very similar to Whatsapp, another popular cross-platform messaging service. Unlike Whatsapp, mysms is opening up its API and will let developers incorporate the service into their own apps.
You can read more about mysms and sign up for a free account on the company's website. You can also download the iPhone app from the iOS App Store. Unfortunately, there is no iPad app, but users can login and send messages via the web app. The mysms service is free, so it's worth checking out if you're looking for a messaging app that supports almost all the devices you'll encounter each day.