If you use an Android phone... Facebook messenger is a great app. You can set it to always on, it doesn't drain the battery and messages are instant.. just like normal text messages... and It is free!. There is also Yahoo Messenger.. a bit heavier on battery drain, but does the same job with more functions, such as video calling, file sending etc
Thanks for the response DeltaRomeo - Surely with your suggestion you can only send messages to people on Facebook (which I'm not) and subscribers to Yahoo Messenger (again on my avoid list). With this app you can SMS any mobile, anywhere in the world