SMS integrates with the Mac OS X Address Book, so you can select friends to contact. After you send a message, the widget remembers your friend's carrier so that you don't need to enter it in the future. Long messages are automatically split into multiple parts. You can include your mobile number or e-mail as the reply address of your messages. This lets recipients reply to your message. The widget works with most major carriers in the USA and Canada as well as T-Mobile in Germany.
This program requires Mac OS X 10.4 Tiger.