Go to your wireless service provider's web site and see what it takes to send an SMS message. Especially if you're paying for SMS text messages, it's in your provider's interest to make it easy for someone to send you a message. For instance, AT&T Wireless' instructions (which work for the iPhone) are here:
https://www.wireless.att.com/support...t=KB63037.html
I tried it, and it works like a champ! You write an email, skip the subject and signature lines, put it your text, and send it. Within seconds you get the message!
Some of the "free" web based services might be fine, but I'd be leary of them using the numbers for phone spam. That said, I tried the Sprint Users site (a site I used a lot prior to the iPhone since I was on Sprint) and tried to send an SMS message. It didn't work for me.