We currently use <a href="http://www.mobilemarketing.net/" rel="nofollow">http://www.mobilemarketing.net/</a> on a couple of projects.  Their site looks terrible, but they are fairly cheap and have 'sticky' sessions.  Their support is also very responsive.<p>Textmarks <a href="http://www.textmarks.com/" rel="nofollow">http://www.textmarks.com/</a> is also good but they no longer support sticky sessions.  They also have free messaging if you don't mind a "powered by textmarks" with a link back to their site at the bottom of your SMS.