For cheap international text messaging you can download a java application to your phone which sends messages over your phones GPRS. This works out to be a lot cheaper than sending messages over the network and the only extra costs are data charges (which vary depending on your network provider).
Different services are:
I personally use a service called FishText, which i have found to be the cheapest and most reliable. To text friends in the U.S. it would cost £0.014 which is about $0.02. You can check it out at www.Fishtext.com.