BrowserTexting is a great app that lets you send sms's from your computer via your smartphone. The free service currently (as of December 2011) lets you send up to 20 sms's a day from your computer, which I think is fair.
UPDATE: Browsertexting now includes commercials in the texts and the free edition is therefore not really usable any more..
UPDATE November 06: The app now is a commercial product with only a 30 free trial period. I'd recommend checking out Mightytext which is both better UI wise and is free.