PiggySMS (formerly WebSMSLib) is a nice alternative to send SMS from your carrier's Web site. It automates all the boring stuff. It's easy as the cell phone: compose, click, sent!
It gives you one-click access to your mobile contacts from the Address Book, remember recents destination and link destination numbers with used accounts. PiggySMS also support multiple accounts for each operator and you it supports CAPTCHA's.
PiggySMS is a cocoa-objectivec port of famous JackSMS service for Windows/Java phones. It will also be available soon for iPhone.
It can be extended to support new carriers writing a plugin. A thoroughly commented template lets you create your own plugin in a minute.
It supports over 40+ service providers!