I've seen SMS widgets for Dashboard come and go as they all have some strange catch, like they only work with a specific carrier or you have to pay a service fee through a specific site.  However this morning I finally found the one - it works! You don't have to sign up at a site, (Although it looks like this one uses a site by Sanity Software to send them) and it can send messages to Cingular, Sprint, Verizon, T-Mobile, Nextel, Virgin Mobile, Alltel, and Qwest in the USA, and they're testing T-Mobile in Germany. What's even better is the widget integrates with Address Book, and once you chose a contact (or enter a number) and chose their carrier - the widget remembers! As icing on the cake: you can include your mobile # or email so your recipient can easily reply. Very slick and it works wonderfully. There's no registration, though the author asks for donations. Go snag a copy!