I have yet to try it out, but Clickatell seems like a nice, developer-friendly SMS gateway.

UPDATE:

They offer a trial account where you get a few messages for free (although they override the message content with a default text), so I went ahead and tried it out.

It’s really a matter of minutes - if not seconds - to get started:

schmidp@dna:~$ curl -i "http://api.clickatell.com/http/sendmsg?user=$USER&password=$PASSWORD&api_id=$APIID &to=$TO&from=$FROM&text=$TEXT"

/HTTP/1.1 200 OK
Date: Tue, 23 Jun 2009 11:59:22 GMT
Server: Apache/2.2.3 (Red Hat)
X-Powered-By: PHP/5.2.6
Connection: close
Content-Type: text/html

ID: 352178af3b24f03ef83997de1024eead
schmidp@dna:~$

I haven’t compared their prices with other gateway providers since I don’t have a real need for it right now.