Depending on the load requirement, I have used NowSMS (nowsms.com) for some of my projects. NowSMS is pretty straight forward to use. For others, I wrote my own

Depending on the load requirement, I have used NowSMS (nowsms.com) for some of my projects. NowSMS is pretty straight forward to use. For others, I wrote my own gateway using Smslib - that can be a pain though, depending on the phone you want to you.