I recall some sites did that. Freebies. I don't know if they still do. I suspect contracts with inclusive may have made them all but redundant, but best of lock with your web search.
I remember using this service aaaages ago so I thought I'd try and see if any are still up and running.
I've just found one and tried it and it works - just sent a text from my laptop to my phone. There's the option to reply to the text, I didn't try it, but I assume that works fine as well:
http://www.cbfsms.com/