WebSMSLib for Mac

From malcom-mac software:

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. WebSMSLib also support multiple accounts for each operator and you it supports CAPTCHA's.

WebSMSLib is a cocoa-objectivec port of famous WebSMS Widget for your dashboard. It will also be available soon for iPhone.

Released under an open source license, it can be extended to support new carriers writing a plugin. A thoroughly commented template lets you create your own plugin in a minute (or you can convert existing WebSMS Widget plugins).

For an updated list of supported carriers take a look here: http://www.malcom-mac.com/blog/websmslib/websmslib-available-plugins/