https://www.vyke.com

Vyke Lite is a Java-based mobile application that allows users to quickly and easily make low cost, operator independent international calls and text messages. It features an ease-of-use oriented interface similar to the scroll-and-click menus found in many mobile handsets. Vyke Lite allows users to access their handset’s address book directly from the application, simplifying the calling and texting process, while also supporting such standard phone features such as predictive text entry.
While Vyke is available on many phones, AFAIK it is not on the iPhone. I feel this would be a brilliant app as you can send text messages for 2 cents / 1 pence and make very cheap phone calls (VOIP) if you are near WiFi.

While I can use the website to send texts on my iPhone, it is quite a slow loading website (on EDGE) so an application would be much handier.

I am based in the UK.