http://www.trinketsoftware.com/PowerSMS/
PowerSMS is a mobile application for people who love text messaging. It enables a number of interesting communication scenarios, which are either cumbersome or impossible without it.
PowerSMS does not replace your phone's built-in text messaging features. Instead, it works with the same folders and messages you already have. It's simple, performs tasks quickly, and gets out of the way. Use PowerSMS to improve communications with your family, friends and colleagues, and they'll wonder how you do it!