The main problem with this widget is that it does not work with all mobile phones. You can find a list of those phones that work and those that do not on the authors website by following this link here.
If you can use it, you just have to write your text in the box and select the phone you want to send it from in the dropdown list.
BlueTooth Texter also includes a small database where your sent messages are saved along with the number of the phone you sent it to.
For Mac OS X 10.4 and above.