Sending SMS - The best option for a startup?

I'm wondering if anyone's got any advice on the most affordable way to send SMS messages. I know there's a lot of providers out there, but if anyone has any advice on specific providers, I'd appreciate it.

I would need the ability to send messages under a couple different numbers/names, and in the beginning I'd only need a couple thousand SMSes a month, but over time, I'd (hopefully!) need a lot more.

Thanks in advance for any ideas....