UPDATE:
Well a new service called Number Guru launch and they offer a developers API, not sure if it's free
- http://www.numberguru.com/developer
OLD:
I would suggest OpenMarket as a SMS aggregator. They offer Number Lookups as part of their service if you needed two-way communication with a Short Code.
If you just need to send a one-way SMS/Text message (Like alerts) but the user would have to opt-out via email and then be removed from your service. You could use something like
Neustar number verification LEAP to do a number lookup (Which displays the carrier information) and use the SMS Email Gateway to send the SMS messages.
