TextGate is a great tool if you write a lot of SMS and perhaps sit by a computer for work or at school. Simply fire up the TextGate app on your phone, then login to the TextGate web interface and start writing and receiving text messages through your web browser instead of using your phone's smaller and more difficult keyboard to write them.
It's very easy and it's completely free (except for the data traffic and SMS fees that you normally have).
TextGate polls the TextGate server with the chosen interval (can be chosen from the menu in the app), and if there are new messages to send, the TextGate app will send them using your phone and your phone number. So it will be just like if you sent the SMS from your phone, with the only difference that you actually typed it into your web browser instead.