The Send SMS widget is a Dashboard widget that can send an SMS on its own. Use your Mac to enter SMS text and send the SMS through the Internet. You don't even need a mobile phone.

SMS Mac integrates seamlessly with your Mac Address Book. With the SMS Mac Address Book plugin installed, you simply open the Address Book card of your friend or contact, click the mobile label and choose the command Send SMS from the popup menu. You can also select a group of contacts and send an SMS to all members of the group!

SMS Mac also supports AppleScript. The SMS Mac Scripting application allows you to send an SMS when a specific event occurs. You can use it, for example, with iCal. You can configure iCal and SMS Mac to send yourself an SMS to remind you of an important meeting. On a Mac OS X server, you can use it to send the system administrator an SMS alert when a certain condition occurs. Several example scripts are included.

SMS Mac comes in several flavors. This download is for the Dashboard widget. SMS Mac is an Address Book plug-in.
A Yahoo! widget(Mac OS X 10.3 or higher, Windows XP, Windows 2000) is available at the author's download page.
The iPhone, iPad and iPod touch version of SMS Mac, SMS touch, is available from the App Store.

Requirements:
OS X 10.4-10.7
Universal Binary