The following is a list of most the SMSC's (Short Message Service Centers) around the world. These can be used to send text messages for free.
All you need to do is change the service center number on your phone (under "message settings") to one of the networks below. Don't choose your own network as you will get charged for it.
There are just a few things to remember when trying this:
If you use a network outside of your country, you will need international call roaming enabled (speak to your service provider about this).
The network you are using may not be able to send to the network you are trying to send to. Therefore it may accept the message but can't send it.
If you use a network outside your own country you will need type the recipients phone number in international format for your country because it's being sent abroad.
For example we are in the UK and to send someone we know a message for free using a forein service center we would type in the ANDORRA message center into our phone and send it to +447788123456.
The networks are becoming wise to this and are barring you from using their SMSC if your not on their network. Therefore it could become time consuming to find one that works.
Remember though, when you find one that works, keep it to yourself because the less people know about it, the better.