I use
http://www.intellisms.co.uk/ (and there are many other services like this).
You buy a fixed number of SMS messages. You can then send these by logging into their web site or by sending a specially formatted email message. A return SMS can either be emailed to you, or you can send a text as if it came from your regular phone (so replies come to your phone directly).
It works well. I've used it a few times when I've wanted to create small apps that notified me of stuff by SMS. Also used it to write longer texts to friends (rather than typing on a tiny screen).